Previously we would blindly emit an sequence like:

        mov(1)          f0.1<1>UW       g1.14<0,1,0>UW
        ...
        cmp.l.f0(16)    g7<1>F          g5<8,8,1>F      0x41700000F  /* 15F */
(+f0.1) cmp.z.f0.1(16)  null<1>D        g7<8,8,1>D      0D

The first move sets the flags based on the initial execution mask.
Later discard sequences contain a predicated compare that can only
remove more SIMD channels.  Often times the only user of the result from
the first compare is the second compare.  Instead, generate a sequence
like

        mov(1)          f0.1<1>UW       g1.14<0,1,0>UW
        ...
        cmp.l.f0(16)    g7<1>F          g5<8,8,1>F      0x41700000F  /* 15F */
(+f0.1) cmp.ge.f0.1(8)  null<1>F        g5<8,8,1>F      0x41700000F  /* 15F */

If the results stored in g7 and f0.0 are not used, the comparison will
be eliminated.  This removes an instruction and potentially reduces
register pressure.

v2: Major re-write of the commit message (including fixing the assembly
code).  Suggested by Matt.

All Gen8+ platforms had similar results. (Ice Lake shown)
total instructions in shared programs: 17224434 -> 17198659 (-0.15%)
instructions in affected programs: 2908125 -> 2882350 (-0.89%)
helped: 18891
HURT: 5
helped stats (abs) min: 1 max: 12 x̄: 1.38 x̃: 1
helped stats (rel) min: 0.03% max: 25.00% x̄: 1.76% x̃: 1.02%
HURT stats (abs)   min: 9 max: 105 x̄: 51.40 x̃: 35
HURT stats (rel)   min: 0.43% max: 4.92% x̄: 2.34% x̃: 1.56%
95% mean confidence interval for instructions value: -1.39 -1.34
95% mean confidence interval for instructions %-change: -1.79% -1.73%
Instructions are helped.

total cycles in shared programs: 361468458 -> 361170679 (-0.08%)
cycles in affected programs: 38470116 -> 38172337 (-0.77%)
helped: 16202
HURT: 1456
helped stats (abs) min: 1 max: 4473 x̄: 26.24 x̃: 18
helped stats (rel) min: <.01% max: 28.44% x̄: 2.90% x̃: 2.18%
HURT stats (abs)   min: 1 max: 5982 x̄: 87.51 x̃: 28
HURT stats (rel)   min: <.01% max: 51.29% x̄: 5.48% x̃: 1.64%
95% mean confidence interval for cycles value: -18.24 -15.49
95% mean confidence interval for cycles %-change: -2.26% -2.14%
Cycles are helped.

total spills in shared programs: 12147 -> 12176 (0.24%)
spills in affected programs: 175 -> 204 (16.57%)
helped: 8
HURT: 5

total fills in shared programs: 25262 -> 25292 (0.12%)
fills in affected programs: 269 -> 299 (11.15%)
helped: 8
HURT: 5

Haswell
total instructions in shared programs: 13530316 -> 13502647 (-0.20%)
instructions in affected programs: 2507824 -> 2480155 (-1.10%)
helped: 18859
HURT: 10
helped stats (abs) min: 1 max: 12 x̄: 1.48 x̃: 1
helped stats (rel) min: 0.03% max: 27.78% x̄: 2.38% x̃: 1.41%
HURT stats (abs)   min: 5 max: 39 x̄: 25.70 x̃: 31
HURT stats (rel)   min: 0.22% max: 1.66% x̄: 1.09% x̃: 1.31%
95% mean confidence interval for instructions value: -1.49 -1.44
95% mean confidence interval for instructions %-change: -2.42% -2.34%
Instructions are helped.

total cycles in shared programs: 377865412 -> 377639034 (-0.06%)
cycles in affected programs: 40169572 -> 39943194 (-0.56%)
helped: 15550
HURT: 1938
helped stats (abs) min: 1 max: 2482 x̄: 25.67 x̃: 18
helped stats (rel) min: <.01% max: 37.77% x̄: 3.00% x̃: 2.25%
HURT stats (abs)   min: 1 max: 4862 x̄: 89.17 x̃: 35
HURT stats (rel)   min: <.01% max: 67.67% x̄: 6.16% x̃: 2.75%
95% mean confidence interval for cycles value: -14.42 -11.47
95% mean confidence interval for cycles %-change: -2.05% -1.91%
Cycles are helped.

total spills in shared programs: 26769 -> 26814 (0.17%)
spills in affected programs: 826 -> 871 (5.45%)
helped: 9
HURT: 10

total fills in shared programs: 38383 -> 38425 (0.11%)
fills in affected programs: 834 -> 876 (5.04%)
helped: 9
HURT: 10

LOST:   5
GAINED: 10

Ivy Bridge
total instructions in shared programs: 12079250 -> 12044139 (-0.29%)
instructions in affected programs: 2409680 -> 2374569 (-1.46%)
helped: 16135
HURT: 0
helped stats (abs) min: 1 max: 23 x̄: 2.18 x̃: 2
helped stats (rel) min: 0.07% max: 37.50% x̄: 2.72% x̃: 1.68%
95% mean confidence interval for instructions value: -2.21 -2.14
95% mean confidence interval for instructions %-change: -2.76% -2.67%
Instructions are helped.

total cycles in shared programs: 180116747 -> 179900405 (-0.12%)
cycles in affected programs: 25439823 -> 25223481 (-0.85%)
helped: 13817
HURT: 1499
helped stats (abs) min: 1 max: 1886 x̄: 26.40 x̃: 18
helped stats (rel) min: <.01% max: 38.84% x̄: 2.57% x̃: 1.97%
HURT stats (abs)   min: 1 max: 3684 x̄: 98.99 x̃: 52
HURT stats (rel)   min: <.01% max: 97.01% x̄: 6.37% x̃: 3.42%
95% mean confidence interval for cycles value: -15.68 -12.57
95% mean confidence interval for cycles %-change: -1.77% -1.63%
Cycles are helped.

LOST:   8
GAINED: 10

Sandy Bridge
total instructions in shared programs: 10878990 -> 10863659 (-0.14%)
instructions in affected programs: 1806702 -> 1791371 (-0.85%)
helped: 13023
HURT: 0
helped stats (abs) min: 1 max: 5 x̄: 1.18 x̃: 1
helped stats (rel) min: 0.07% max: 13.79% x̄: 1.65% x̃: 1.10%
95% mean confidence interval for instructions value: -1.18 -1.17
95% mean confidence interval for instructions %-change: -1.68% -1.62%
Instructions are helped.

total cycles in shared programs: 154082878 -> 153862810 (-0.14%)
cycles in affected programs: 20199374 -> 19979306 (-1.09%)
helped: 12048
HURT: 510
helped stats (abs) min: 1 max: 323 x̄: 20.57 x̃: 18
helped stats (rel) min: 0.03% max: 17.78% x̄: 2.05% x̃: 1.52%
HURT stats (abs)   min: 1 max: 448 x̄: 54.39 x̃: 16
HURT stats (rel)   min: 0.02% max: 37.98% x̄: 4.13% x̃: 1.17%
95% mean confidence interval for cycles value: -17.97 -17.08
95% mean confidence interval for cycles %-change: -1.84% -1.75%
Cycles are helped.

LOST:   1
GAINED: 0

Iron Lake
total instructions in shared programs: 8155075 -> 8142729 (-0.15%)
instructions in affected programs: 949495 -> 937149 (-1.30%)
helped: 5810
HURT: 0
helped stats (abs) min: 1 max: 8 x̄: 2.12 x̃: 2
helped stats (rel) min: 0.10% max: 16.67% x̄: 2.53% x̃: 1.85%
95% mean confidence interval for instructions value: -2.14 -2.11
95% mean confidence interval for instructions %-change: -2.59% -2.48%
Instructions are helped.

total cycles in shared programs: 188584610 -> 188549632 (-0.02%)
cycles in affected programs: 17274446 -> 17239468 (-0.20%)
helped: 3881
HURT: 90
helped stats (abs) min: 2 max: 168 x̄: 9.08 x̃: 6
helped stats (rel) min: <.01% max: 23.53% x̄: 0.83% x̃: 0.30%
HURT stats (abs)   min: 2 max: 10 x̄: 2.80 x̃: 2
HURT stats (rel)   min: <.01% max: 0.60% x̄: 0.10% x̃: 0.07%
95% mean confidence interval for cycles value: -9.35 -8.27
95% mean confidence interval for cycles %-change: -0.85% -0.77%
Cycles are helped.

GM45
total instructions in shared programs: 5019308 -> 5013119 (-0.12%)
instructions in affected programs: 489028 -> 482839 (-1.27%)
helped: 2912
HURT: 0
helped stats (abs) min: 1 max: 8 x̄: 2.13 x̃: 2
helped stats (rel) min: 0.10% max: 16.67% x̄: 2.46% x̃: 1.81%
95% mean confidence interval for instructions value: -2.14 -2.11
95% mean confidence interval for instructions %-change: -2.54% -2.39%
Instructions are helped.

total cycles in shared programs: 129002592 -> 128977804 (-0.02%)
cycles in affected programs: 12669152 -> 12644364 (-0.20%)
helped: 2759
HURT: 37
helped stats (abs) min: 2 max: 168 x̄: 9.03 x̃: 4
helped stats (rel) min: <.01% max: 21.43% x̄: 0.75% x̃: 0.31%
HURT stats (abs)   min: 2 max: 10 x̄: 3.62 x̃: 4
HURT stats (rel)   min: <.01% max: 0.41% x̄: 0.10% x̃: 0.04%
95% mean confidence interval for cycles value: -9.53 -8.20
95% mean confidence interval for cycles %-change: -0.79% -0.70%
Cycles are helped.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>