[X86][SSE] Enable commutation for SSE immediate blend instructions

Patch to allow (v)blendps, (v)blendpd, (v)pblendw and vpblendd instructions to be commuted - swaps the src registers and inverts the blend mask.

This is primarily to improve memory folding (see new tests), but it also improves the quality of shuffles (see modified tests).

Differential Revision: http://reviews.llvm.org/D6015