[X86][SSE] pslldq/psrldq shuffle mask decodes

Patch to provide shuffle decodes and asm comments for the sse pslldq/psrldq SSE2/AVX2 byte shift instructions.

Differential Revision: http://reviews.llvm.org/D5598


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219738 91177308-0d34-0410-b5e6-96231b3b80d8