i965/fs: don't propagate cmod when the exec sizes differ

i965/fs: don't propagate cmod when the exec sizes differ

This can happen when the source of the compare was split by the SIMD
lowering pass. Potentially, we could allow the case where the exec size
of scan_inst is larger, and scan_inst has the right quarter selected,
but doing that seems a little more risky.

v2: Merge the bail condition into the the previous if/break block (Matt)

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>