[X86] Emit a PACKUS instead of a VECTOR_SHUFFLE from LowerTRUNCATE for v16i16->v16i8.

We can't guarantee that demanded bits passing through the vector shuffle won't cause the AND in front of this to be removed. This would prevent the PACKUS from being matched during shuffle lowering.

Unfortunately, this adds a packuswb to one of the vector-reduce-mul.ll tests since we were removing the shuffle via SimplifyDemandedVectorElts. We appear to have similar issues with vpmovwb on the same test case on other targets.

llvm-svn: 347361