intel/compiler: Use partial redundancy elimination for compares

intel/compiler: Use partial redundancy elimination for compares

Almost all of the hurt shaders are repeated instances of the same shader
in synmark's compilation speed tests.

shader-db results:

All Gen6+ platforms had similar results. (Skylake shown)
total instructions in shared programs: 15256840 -> 15256389 (<.01%)
instructions in affected programs: 54137 -> 53686 (-0.83%)
helped: 288
HURT: 0
helped stats (abs) min: 1 max: 15 x̄: 1.57 x̃: 1
helped stats (rel) min: 0.06% max: 26.67% x̄: 1.99% x̃: 0.74%
95% mean confidence interval for instructions value: -1.76 -1.38
95% mean confidence interval for instructions %-change: -2.47% -1.50%
Instructions are helped.

total cycles in shared programs: 372286583 -> 372283851 (<.01%)
cycles in affected programs: 833829 -> 831097 (-0.33%)
helped: 265
HURT: 16
helped stats (abs) min: 2 max: 74 x̄: 11.81 x̃: 4
helped stats (rel) min: 0.04% max: 9.07% x̄: 0.99% x̃: 0.35%
HURT stats (abs)   min: 2 max: 130 x̄: 24.88 x̃: 8
HURT stats (rel)   min: <.01% max: 12.31% x̄: 1.44% x̃: 0.27%
95% mean confidence interval for cycles value: -12.30 -7.15
95% mean confidence interval for cycles %-change: -1.06% -0.64%
Cycles are helped.

Iron Lake and GM45 had similar results. (GM45 shown)
total instructions in shared programs: 5038653 -> 5038495 (<.01%)
instructions in affected programs: 13939 -> 13781 (-1.13%)
helped: 50
HURT: 1
helped stats (abs) min: 1 max: 15 x̄: 3.18 x̃: 4
helped stats (rel) min: 0.33% max: 13.33% x̄: 2.24% x̃: 1.09%
HURT stats (abs)   min: 1 max: 1 x̄: 1.00 x̃: 1
HURT stats (rel)   min: 0.83% max: 0.83% x̄: 0.83% x̃: 0.83%
95% mean confidence interval for instructions value: -3.73 -2.47
95% mean confidence interval for instructions %-change: -3.16% -1.21%
Instructions are helped.

total cycles in shared programs: 128118922 -> 128118228 (<.01%)
cycles in affected programs: 134906 -> 134212 (-0.51%)
helped: 50
HURT: 0
helped stats (abs) min: 2 max: 60 x̄: 13.88 x̃: 18
helped stats (rel) min: 0.06% max: 3.19% x̄: 0.74% x̃: 0.70%
95% mean confidence interval for cycles value: -16.54 -11.22
95% mean confidence interval for cycles %-change: -0.95% -0.53%
Cycles are helped.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>