i965/vec4: Allow CSE on subset VF constant loads

i965/vec4: Allow CSE on subset VF constant loads

v2: Rewrite the code that generates the VF mask.  Suggested by Ken.

No changes on other platforms.

Haswell, Ivy Bridge, and Sandy Bridge had similar results. (Haswell shown)
total instructions in shared programs: 13059891 -> 13059884 (<.01%)
instructions in affected programs: 431 -> 424 (-1.62%)
helped: 7
HURT: 0
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 1.19% max: 5.26% x̄: 2.05% x̃: 1.49%
95% mean confidence interval for instructions value: -1.00 -1.00
95% mean confidence interval for instructions %-change: -3.39% -0.71%
Instructions are helped.

total cycles in shared programs: 409260032 -> 409260018 (<.01%)
cycles in affected programs: 4228 -> 4214 (-0.33%)
helped: 7
HURT: 0
helped stats (abs) min: 2 max: 2 x̄: 2.00 x̃: 2
helped stats (rel) min: 0.28% max: 2.04% x̄: 0.54% x̃: 0.28%
95% mean confidence interval for cycles value: -2.00 -2.00
95% mean confidence interval for cycles %-change: -1.15% 0.07%

Inconclusive result (%-change mean confidence interval includes 0).

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>