[X86][SSE] Improve legal SHUFP and PSHUFD shuffle matching

Updated X86TargetLowering::isShuffleMaskLegal to match SHUFP masks with commuted inputs and PSHUFD masks that reference the second input.

As part of this I've refactored isPSHUFDMask to work in a more general manner and allow it to match against either the first or second input vector.

Differential Revision: http://reviews.llvm.org/D6287