On vector platforms, this helps elide some constant loads.

v2: Reorder the transformations.

No changes on Broadwell or Skylake.

Haswell
total instructions in shared programs: 13093793 -> 13060163 (-0.26%)
instructions in affected programs: 1277532 -> 1243902 (-2.63%)
helped: 13216
HURT: 95
helped stats (abs) min: 1 max: 18 x̄: 2.56 x̃: 2
helped stats (rel) min: 0.21% max: 20.00% x̄: 3.63% x̃: 2.78%
HURT stats (abs)   min: 1 max: 6 x̄: 1.77 x̃: 1
HURT stats (rel)   min: 0.09% max: 5.56% x̄: 1.25% x̃: 1.19%
95% mean confidence interval for instructions value: -2.57 -2.49
95% mean confidence interval for instructions %-change: -3.65% -3.54%
Instructions are helped.

total cycles in shared programs: 409580819 -> 409268463 (-0.08%)
cycles in affected programs: 71730652 -> 71418296 (-0.44%)
helped: 9898
HURT: 2352
helped stats (abs) min: 2 max: 16014 x̄: 37.08 x̃: 16
helped stats (rel) min: <.01% max: 35.55% x̄: 6.26% x̃: 4.50%
HURT stats (abs)   min: 2 max: 276 x̄: 23.25 x̃: 6
HURT stats (rel)   min: <.01% max: 40.00% x̄: 3.54% x̃: 1.97%
95% mean confidence interval for cycles value: -33.19 -17.80
95% mean confidence interval for cycles %-change: -4.50% -4.26%
Cycles are helped.

total fills in shared programs: 82059 -> 82052 (<.01%)
fills in affected programs: 21 -> 14 (-33.33%)
helped: 7
HURT: 0

Sandy Bridge and Ivy Bridge had similar results (Ivy Bridge shown)
total instructions in shared programs: 11811851 -> 11780605 (-0.26%)
instructions in affected programs: 1155007 -> 1123761 (-2.71%)
helped: 12304
HURT: 95
helped stats (abs) min: 1 max: 18 x̄: 2.55 x̃: 2
helped stats (rel) min: 0.21% max: 20.00% x̄: 3.69% x̃: 2.86%
HURT stats (abs)   min: 1 max: 6 x̄: 1.77 x̃: 1
HURT stats (rel)   min: 0.09% max: 5.56% x̄: 1.25% x̃: 1.19%
95% mean confidence interval for instructions value: -2.56 -2.48
95% mean confidence interval for instructions %-change: -3.71% -3.59%
Instructions are helped.

total cycles in shared programs: 257618409 -> 257316805 (-0.12%)
cycles in affected programs: 71999580 -> 71697976 (-0.42%)
helped: 9155
HURT: 2380
helped stats (abs) min: 2 max: 16014 x̄: 38.44 x̃: 16
helped stats (rel) min: <.01% max: 35.75% x̄: 6.39% x̃: 4.62%
HURT stats (abs)   min: 2 max: 290 x̄: 21.14 x̃: 4
HURT stats (rel)   min: <.01% max: 41.55% x̄: 3.14% x̃: 1.33%
95% mean confidence interval for cycles value: -34.32 -17.97
95% mean confidence interval for cycles %-change: -4.55% -4.29%
Cycles are helped.

GM45 and Iron Lake had nearly identical results (Iron Lake shown)
total instructions in shared programs: 7886750 -> 7879944 (-0.09%)
instructions in affected programs: 373781 -> 366975 (-1.82%)
helped: 3715
HURT: 47
helped stats (abs) min: 1 max: 8 x̄: 1.86 x̃: 1
helped stats (rel) min: 0.22% max: 16.67% x̄: 2.88% x̃: 2.06%
HURT stats (abs)   min: 1 max: 6 x̄: 2.55 x̃: 2
HURT stats (rel)   min: 1.09% max: 5.00% x̄: 1.93% x̃: 2.35%
95% mean confidence interval for instructions value: -1.85 -1.77
95% mean confidence interval for instructions %-change: -2.91% -2.73%
Instructions are helped.

total cycles in shared programs: 178114636 -> 178095452 (-0.01%)
cycles in affected programs: 7227666 -> 7208482 (-0.27%)
helped: 3349
HURT: 301
helped stats (abs) min: 2 max: 90 x̄: 6.55 x̃: 4
helped stats (rel) min: <.01% max: 14.18% x̄: 0.95% x̃: 0.63%
HURT stats (abs)   min: 2 max: 42 x̄: 9.13 x̃: 10
HURT stats (rel)   min: 0.01% max: 11.19% x̄: 1.22% x̃: 1.50%
95% mean confidence interval for cycles value: -5.52 -4.99
95% mean confidence interval for cycles %-change: -0.81% -0.73%
Cycles are helped.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com> [v1]