[X86][SSE] combineX86ShuffleChain - attempt to recognise 'hidden' identity shuffles

See if the combined shuffle mask is equivalent to an identity shuffle, typically this is due to repeated LHS/RHS ops in horiz-ops, but isTargetShuffleEquivalent might see other patterns as well.

This is another small step towards getting rid of foldShuffleOfHorizOp and relying on canonicalizeShuffleMaskWithHorizOp and generic shuffle combining.