[CostModel] getInstructionCost - improve estimation of costs for length changing shuffles (#84156)

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)