[X86][SSE] Use raw shuffle mask decode in SimplifyDemandedVectorEltsForTargetNode (PR39549)

We were using the 'normalized' shuffle mask from resolveTargetShuffleInputs, which replaces zero/undef inputs with sentinel values. For SimplifyDemandedVectorElts we need the raw mask so we can correctly demand those 'zero' inputs that got normalized away, this requires an extra bit of logic to locally normalize undef inputs.

llvm-svn: 347158