[X86] lowerShuffleAsDecomposedShuffleMerge - try to match...

[X86] lowerShuffleAsDecomposedShuffleMerge - try to match unpck(permute(x),permute(y)) for v4i32/v2i64 shuffles

We're using lowerShuffleAsPermuteAndUnpack, which can probably be improved to handle 256/512-bit types pretty easily.

First step towards trying to address the poor vector-shuffle-sse4a.ll pre-SSSE3 codegen mentioned on D127115