[CostModel][X86][ARM] Teach getCastInstrCost to include the splitting factor...

[CostModel][X86][ARM] Teach getCastInstrCost to include the splitting factor when handling operations that type legalize to the same number of subvectors or scalar components

Previously, we just always returned 1. But that ignores that we have to do the operation for each subvector or scalar component.

Differential Revision: https://reviews.llvm.org/D78824