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

i965/fs: 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 could reproduce this easily with SIMD32 but I don't see any reason
why the problem would be SIMD32-specific, it was most likely working
by luck.

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