[CostModel][X86] getCastInstrCost - improve CostKind adjustment when splitting src/dst types

Noticed in #90883 review - for non-Throughput costs, we weren't applying the split count to '0 or 1' cost value.

This still doesn't work well as many of the type legalizations are hidden so we don't have the split count, really we need to move a CostKindCosts based costs table, but that's going to be a lot of work :/