v2: Fix several problems handling inverted predicates.  Add a much
bigger comment around the BRW_CONDITIONAL_NZ case.

v3: Allow uniforms and shader inputs as sources for the original SEL and
CMP instructions.  This enables a LOT more shaders to receive CSEL
merging (5816 vs 8564 on SKL).

v4: Report progress.

Broadwell and Skylake had similar results. (Broadwell shown)
helped: 8527
HURT: 0
helped stats (abs) min: 1 max: 27 x̄: 2.44 x̃: 1
helped stats (rel) min: 0.03% max: 17.80% x̄: 1.12% x̃: 0.70%
95% mean confidence interval for instructions value: -2.51 -2.36
95% mean confidence interval for instructions %-change: -1.15% -1.10%
Instructions are helped.

total cycles in shared programs: 559442317 -> 558288357 (-0.21%)
cycles in affected programs: 372699860 -> 371545900 (-0.31%)
helped: 6748
HURT: 1450
helped stats (abs) min: 1 max: 32000 x̄: 182.41 x̃: 12
helped stats (rel) min: <.01% max: 66.08% x̄: 3.42% x̃: 0.70%
HURT stats (abs)   min: 1 max: 2538 x̄: 53.08 x̃: 14
HURT stats (rel)   min: <.01% max: 96.72% x̄: 3.32% x̃: 0.90%
95% mean confidence interval for cycles value: -179.01 -102.51
95% mean confidence interval for cycles %-change: -2.37% -2.08%
Cycles are helped.

LOST:   0
GAINED: 6

No changes on earlier platforms.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com> [v1]
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> [v3]
Reviewed-by: Matt Turner <mattst88@gmail.com>