[X86] Fold PSHUF(VSHIFT(X,Y)) -> VSHIFT(PSHUF(X),Y)

PSHUFD/PSHUFLW/PSHUFHW can act as a vector move / folded load, notably helping simplify pre-AVX cases in particular.

This is a much milder alternative to refactoring canonicalizeShuffleWithBinOps to support SSE shifts nodes.