Add X86 code emitter support AVX encoded MRMDestReg instructions.

Previously we weren't skipping the VVVV encoded register. Based on patch by Michael Liao.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177221 91177308-0d34-0410-b5e6-96231b3b80d8