[X86] Use shufflevector instead of a select with a constant mask for fmaddsub/fmsubad...

[X86] Use shufflevector instead of a select with a constant mask for fmaddsub/fmsubadd IR emission.

Shufflevector is easier to generate and matches what the backend pattern matches without relying on constant selects being turned into shuffles.

While I was there I also made the IR regular expressions a little stricter to ensure operand order on the shuffle.

llvm-svn: 336388