[X86] When lowering v32i8 MULHS/MULHU, shuffle after the PACKUS rather than before.

We're using a 256-bit PACKUS to do the truncation, but that instruction operates on 128-bit lanes. So previously we shuffled first to rearrange the lanes. But that requires 2 shuffles. Instead we can shuffle after the PACKUS using a single VPERMQ. This matches what our normal LowerTRUNCATE code does when it uses PACKUS.

Differential Revision: https://reviews.llvm.org/D51284

llvm-svn: 340757