i965/vec4: Don't cmod propagate from CMP to ADD if the writemask isn't compatible

Otherwise we can incorrectly cmod propagate in situations like

    add(8)          g10<1>.xD       g2<0>.xD        -16D
    ...
    cmp.ge.f0(8)    null<1>D        g2<0>.xD        16D
    ...
    (+f0) sel(8)    g21<1>.xyUD     g14<4>.xyyyUD   g18<4>.xyyyUD

Sadly, this change hurts quite a few shaders.

v2: Refactor writemask compatibility check into a separate function.
Suggested by Caio.

Ivy Bridge and Haswell had similar results. (Haswell shown)
total instructions in shared programs: 12968489 -> 12968738 (<.01%)
instructions in affected programs: 60679 -> 60928 (0.41%)
helped: 0
HURT: 249
HURT stats (abs)   min: 1 max: 1 x̄: 1.00 x̃: 1
HURT stats (rel)   min: 0.22% max: 0.81% x̄: 0.46% x̃: 0.44%
95% mean confidence interval for instructions value: 1.00 1.00
95% mean confidence interval for instructions %-change: 0.44% 0.48%
Instructions are HURT.

total cycles in shared programs: 409171965 -> 409172317 (<.01%)
cycles in affected programs: 260056 -> 260408 (0.14%)
helped: 0
HURT: 176
HURT stats (abs)   min: 2 max: 2 x̄: 2.00 x̃: 2
HURT stats (rel)   min: 0.04% max: 0.34% x̄: 0.17% x̃: 0.17%
95% mean confidence interval for cycles value: 2.00 2.00
95% mean confidence interval for cycles %-change: 0.16% 0.18%
Cycles are HURT.

Sandy Bridge
total instructions in shared programs: 10423577 -> 10423753 (<.01%)
instructions in affected programs: 40667 -> 40843 (0.43%)
helped: 0
HURT: 176
HURT stats (abs)   min: 1 max: 1 x̄: 1.00 x̃: 1
HURT stats (rel)   min: 0.29% max: 0.79% x̄: 0.48% x̃: 0.42%
95% mean confidence interval for instructions value: 1.00 1.00
95% mean confidence interval for instructions %-change: 0.46% 0.51%
Instructions are HURT.

total cycles in shared programs: 146097503 -> 146097855 (<.01%)
cycles in affected programs: 503990 -> 504342 (0.07%)
helped: 0
HURT: 176
HURT stats (abs)   min: 2 max: 2 x̄: 2.00 x̃: 2
HURT stats (rel)   min: 0.02% max: 0.36% x̄: 0.12% x̃: 0.11%
95% mean confidence interval for cycles value: 2.00 2.00
95% mean confidence interval for cycles %-change: 0.11% 0.13%
Cycles are HURT.

No changes on any other platforms.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Fixes: cd635d14 i965/vec4: Propagate conditional modifiers from compares to adds
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
(cherry picked from commit 995d9937)