[X86][XOP] Support for VPPERM 2-input shuffle mask decoding

This patch adds support for decoding XOP VPPERM instruction when it represents a basic shuffle.

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.

Differential Revision: http://reviews.llvm.org/D18441

llvm-svn: 265874