[x86] Run most of the rest of the shuffle combining over non-128-bit

vectors. This lets us fix the rest of the v16 lowering problems when
pshufb is clearly better.

We might still be able to improve some of the lowerings by enabling the
other combine-based rewriting to fire for non-128-bit vectors, but this
at least should remove any regressions from using the fancy v16i16
lowering strategy.

llvm-svn: 230753