Revert rL344933 from llvm/trunk: [X86][SSE] Tidyup DecodeVPERMILPMask shuffle mask decoding

We can't safely assume that certain RawMask entries are UNDEF as most variable shuffles ignore non-index bits.
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Add support for UNDEF raw mask elements and remove the ConstantPool DecodeVPERMILPMask usage in X86ISelLowering.cpp

llvm-svn: 344936