i965/vec4: Fix cmod propagation not to propagate non-identity cmod into CMP(N).

i965/vec4: Fix cmod propagation not to propagate non-identity cmod into CMP(N).

The conditional mod of these instructions determines the semantics of
the comparison itself (rather than being evaluated based on the result
of the instruction as is usually the case for most other instructions
that allow conditional mods), so it's in general not legal to
propagate a conditional mod into a CMP instruction.  This prevents
cmod propagation from (mis)optimizing:

 cmp.z.f0 tmp, ...
 mov.z.f0 null, tmp

into:

 cmp.z.f0 tmp, ...

which gives the negation of the flag result of the original sequence.
I originally noticed this while working on SIMD32 in the scalar
back-end, but the same scenario is likely to be possible in vec4
programs so this commit ports the bugfix with the same name from the
scalar back-end to the vec4 cmod propagation pass.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>