[X86] Consistently use 128 as the PSHUFB/VPPERM index for zero

Bit 7 of the index controls zeroing, the other bits are ignored when bit 7 is set. Shuffle lowering was using 128 and shuffle combining was using 255. Seems like we should be consistent.

This patch changes shuffle combining to use 128 to match lowering.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D83587