nir/algebraic: Strength reduce some compares of x and -x

Converting the x vs -x comparison to an x vs 0 comparison enable cmod
propagation to help.

The seems to be a win everywhere except Gen7.

Skylake and Broadwell had similar results. (Broadwell shown)
total instructions in shared programs: 15566733 -> 15566014 (<.01%)
instructions in affected programs: 72617 -> 71898 (-0.99%)
helped: 302
HURT: 0
helped stats (abs) min: 1 max: 8 x̄: 2.38 x̃: 2
helped stats (rel) min: 0.15% max: 7.69% x̄: 1.28% x̃: 0.98%
95% mean confidence interval for instructions value: -2.55 -2.21
95% mean confidence interval for instructions %-change: -1.40% -1.16%
Instructions are helped.

total cycles in shared programs: 413014786 -> 413015475 (<.01%)
cycles in affected programs: 707594 -> 708283 (0.10%)
helped: 227
HURT: 101
helped stats (abs) min: 1 max: 612 x̄: 36.07 x̃: 20
helped stats (rel) min: 0.04% max: 19.39% x̄: 2.25% x̃: 1.49%
HURT stats (abs)   min: 2 max: 334 x̄: 87.90 x̃: 45
HURT stats (rel)   min: 0.07% max: 14.51% x̄: 4.54% x̃: 3.36%
95% mean confidence interval for cycles value: -8.12 12.32
95% mean confidence interval for cycles %-change: -0.67% 0.34%
Inconclusive result (value mean confidence interval includes 0).

Haswell and Ivy Bridge had similar results. (Haswell shown)
total instructions in shared programs: 13828220 -> 13827881 (<.01%)
instructions in affected programs: 60887 -> 60548 (-0.56%)
helped: 253
HURT: 6
helped stats (abs) min: 1 max: 5 x̄: 1.36 x̃: 1
helped stats (rel) min: 0.16% max: 3.85% x̄: 0.81% x̃: 0.64%
HURT stats (abs)   min: 1 max: 1 x̄: 1.00 x̃: 1
HURT stats (rel)   min: 0.26% max: 0.89% x̄: 0.47% x̃: 0.27%
95% mean confidence interval for instructions value: -1.39 -1.23
95% mean confidence interval for instructions %-change: -0.85% -0.70%
Instructions are helped.

total cycles in shared programs: 386870095 -> 386894412 (<.01%)
cycles in affected programs: 1537307 -> 1561624 (1.58%)
helped: 127
HURT: 188
helped stats (abs) min: 1 max: 381 x̄: 17.89 x̃: 4
helped stats (rel) min: 0.02% max: 14.33% x̄: 1.00% x̃: 0.33%
HURT stats (abs)   min: 2 max: 5585 x̄: 141.43 x̃: 14
HURT stats (rel)   min: 0.03% max: 11.50% x̄: 1.65% x̃: 1.06%
95% mean confidence interval for cycles value: 21.95 132.45
95% mean confidence interval for cycles %-change: 0.32% 0.85%
Cycles are HURT.

Sandy Bridge
total instructions in shared programs: 10896339 -> 10896276 (<.01%)
instructions in affected programs: 10757 -> 10694 (-0.59%)
helped: 49
HURT: 0
helped stats (abs) min: 1 max: 2 x̄: 1.29 x̃: 1
helped stats (rel) min: 0.12% max: 1.85% x̄: 0.87% x̃: 0.89%
95% mean confidence interval for instructions value: -1.42 -1.15
95% mean confidence interval for instructions %-change: -1.03% -0.72%
Instructions are helped.

total cycles in shared programs: 155091003 -> 155090480 (<.01%)
cycles in affected programs: 102761 -> 102238 (-0.51%)
helped: 51
HURT: 0
helped stats (abs) min: 1 max: 36 x̄: 10.25 x̃: 4
helped stats (rel) min: 0.02% max: 2.57% x̄: 0.76% x̃: 0.36%
95% mean confidence interval for cycles value: -12.98 -7.53
95% mean confidence interval for cycles %-change: -0.97% -0.56%
Cycles are helped.

Iron Lake and GM45 had similar results. (Iron Lake shown)
total instructions in shared programs: 8234667 -> 8234652 (<.01%)
instructions in affected programs: 2063 -> 2048 (-0.73%)
helped: 15
HURT: 0
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 0.30% max: 1.56% x̄: 0.82% x̃: 0.81%
95% mean confidence interval for instructions value: -1.00 -1.00
95% mean confidence interval for instructions %-change: -0.97% -0.67%
Instructions are helped.

total cycles in shared programs: 188700906 -> 188700598 (<.01%)
cycles in affected programs: 283480 -> 283172 (-0.11%)
helped: 83
HURT: 3
helped stats (abs) min: 2 max: 8 x̄: 3.78 x̃: 4
helped stats (rel) min: 0.04% max: 0.55% x̄: 0.15% x̃: 0.12%
HURT stats (abs)   min: 2 max: 2 x̄: 2.00 x̃: 2
HURT stats (rel)   min: 0.02% max: 0.04% x̄: 0.03% x̃: 0.04%
95% mean confidence interval for cycles value: -3.87 -3.29
95% mean confidence interval for cycles %-change: -0.16% -0.12%
Cycles are helped.

Reviewed-by: Matt Turner <mattst88@gmail.com>
14 jobs for !27 with fsign-optimization-v2 in 11 minutes and 13 seconds (queued for 1 second)
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