[x86] Fix a moderately terrifying bug in the new 128-bit shuffle logic

that managed to elude all of my fuzz testing historically. =/

Something changed to allow this code path to actually be exercised and
it was doing bad things. It is especially heavily exercised by the
patterns that emerge when doing AVX shuffles that end up lowered through
the 128-bit code path.

llvm-svn: 218540