If there is a CMP.NZ that compares a single component (via a .zzzz
swizzle, for example) with 0, it can propagate its conditional modifier
back to a previous CMP that writes only that component.  The specific
case that I saw was:

    cmp.l.f0(8)     g42<1>.xF       g61<4>.xF       (abs)g18<4>.zF
    ...
    cmp.nz.f0(8)    null<1>D        g42<4>.xD       0D

In this case we can just delete the second CMP.

No changes on Broadwell or Skylake because they do not use the vec4
backend.  Also no changes on GM45 or Iron Lake.

Sandy Bridge, Ivy Bridge, and Haswell had similar results. (Sandy Bridge shown)
total instructions in shared programs: 10856676 -> 10852569 (-0.04%)
instructions in affected programs: 228322 -> 224215 (-1.80%)
helped: 1331
HURT: 0
helped stats (abs) min: 1 max: 7 x̄: 3.09 x̃: 4
helped stats (rel) min: 0.11% max: 6.67% x̄: 1.88% x̃: 1.83%
95% mean confidence interval for instructions value: -3.19 -2.99
95% mean confidence interval for instructions %-change: -1.93% -1.83%
Instructions are helped.

total cycles in shared programs: 154788865 -> 154732047 (-0.04%)
cycles in affected programs: 2485892 -> 2429074 (-2.29%)
helped: 1097
HURT: 59
helped stats (abs) min: 2 max: 168 x̄: 51.96 x̃: 64
helped stats (rel) min: 0.12% max: 12.70% x̄: 3.44% x̃: 2.22%
HURT stats (abs)   min: 2 max: 16 x̄: 3.02 x̃: 2
HURT stats (rel)   min: 0.18% max: 0.83% x̄: 0.64% x̃: 0.71%
95% mean confidence interval for cycles value: -51.04 -47.26
95% mean confidence interval for cycles %-change: -3.40% -3.07%
Cycles are helped.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>