Fix gap in the cost estimation for length changing shuffles, by adjusting the shuffle mask and either widening the shuffle inputs or extracting the lower elements of the result.
A small step towards moving some of this implementation inside improveShuffleKindFromMask and/or target getShuffleCost handlers (and reduce the diffs in cost estimation depending on whether coming from a ShuffleVectorInst or the raw operands / mask components)
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