i965/vec4: Relax writemask condition in CSE

i965/vec4: Relax writemask condition in CSE

If the previously seen instruction generates more fields than the new
instruction, still allow CSE to happen.  This doesn't do much, but it
also enables a couple more shaders in the next patch.  It helped quite a
bit in another change series that I have (at least for now) abandoned.

v2: Add some extra comentary about the parameters to instructions_match.
Suggested by Ken.

No changes on Skylake, Broadwell, Iron Lake or GM45.

Ivy Bridge and Haswell had similar results. (Ivy Bridge shown)
total instructions in shared programs: 11780295 -> 11780294 (<.01%)
instructions in affected programs: 302 -> 301 (-0.33%)
helped: 1
HURT: 0

total cycles in shared programs: 257308315 -> 257308313 (<.01%)
cycles in affected programs: 2074 -> 2072 (-0.10%)
helped: 1
HURT: 0

Sandy Bridge
total instructions in shared programs: 10506687 -> 10506686 (<.01%)
instructions in affected programs: 335 -> 334 (-0.30%)
helped: 1
HURT: 0

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