i965/fs: Fix CSE temporary copy for some LOAD_PAYLOAD corner cases.

i965/fs: Fix CSE temporary copy for some LOAD_PAYLOAD corner cases.

If the LOAD_PAYLOAD instruction only has header sources it's possible
for the number of registers written to be less than or equal to the
SIMD component size, in which case it would take the single-MOV path
at the bottom which would cause the channel enable masks to be applied
incorrectly to the header contents and/or cause it to write past the
end of the allocated temporary.  If the instruction is either
LOAD_PAYLOAD or doesn't write exactly one component the MOV path is
going to mess up the program so just don't use it.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>