Revert rL344931 from llvm/trunk: [X86][SSE] getTargetShuffleMaskIndices -...

Revert rL344931 from llvm/trunk: [X86][SSE] getTargetShuffleMaskIndices - allow opt-in support for whole undef shuffle mask elements
We can't safely assume that certain RawMask entries are UNDEF as most variable shuffles ignore non-index bits - PSHUFB only works on i8 elts so it'd be safe to use but I'm intending to come up with an alternative approach that works for all.
........
Enable this for PSHUFB constant mask decoding and remove the ConstantPool DecodePSHUFBMask

llvm-svn: 344937