This patch begins adding support for lowering to the XOP VPPERM instruction - adding the X86ISD::VPPERM opcode and shuffle mask decoding (for the more basic shuffle operations that the instruction supports).
The mask decoding required the existing MCInstrLowering code to be updated to support binary shuffles - the implementation now matches what is done in X86InstrCOmments.cpp. This should be useful for some AVX512 binary shuffles (VPERMT2 etc.) as well.
A followup patch will enable VPPERM as a target shuffle for combining.