i965/fs: No need to unzip SIMD-periodic sources during SIMD lowering.

i965/fs: No need to unzip SIMD-periodic sources during SIMD lowering.

If the source value is going to the same for all SIMD-lowered chunks
of the instruction there should be no need to unzip the value into
multiple temporary registers one for each lowered chunk.  As a side
effect this fixes SIMD lowering of instructions with a vector
immediate source.  In the long term it *might* still be worth fixing
offset() to handle vector immediates correctly though, this should be
good enough for the moment.

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