Start the support for AVX instructions with 256-bit %ymm registers. A couple of

notes:
- The instructions are being added with dummy placeholder patterns using some 256
  specifiers, this is not meant to work now, but since there are some multiclasses
  generic enough to accept them,  when we go for codegen, the stuff will be already
  there.
- Add VEX encoding bits to support YMM
- Add MOVUPS and MOVAPS in the first round
- Use "Y" as suffix for those Instructions: MOVUPSYrr, ...
- All AVX instructions in X86InstrSSE.td will move soon to a new X86InstrAVX
  file.

llvm-svn: 107996