[X86] SimplifyDemandedVectorElts - attempt to recombine target shuffle using DemandedElts mask

If we don't demand all elements, then attempt to combine to a simpler shuffle.

At the moment we can only do this if Depth == 0 as combineX86ShufflesRecursively uses Depth to track whether the shuffle has really changed or not - we'll need to change this before we can properly start merging combineX86ShufflesRecursively into SimplifyDemandedVectorElts. 

The insertps-combine.ll regression is because XFormVExtractWithShuffleIntoLoad can't see through shuffles of different widths - this will be fixed in a follow-up commit.

llvm-svn: 368307

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