[CostModel][X86][ARM] Teach default implementation of getCastInstrCost to not add a split/join cost if source type and the destination type both have a SplitVector action

If both the source and the destination need to be split then the two halves of the split operation are completely independent and don't need to be split or joined. So we don't need to assess a cost for the split or join.

Differential Revision: https://reviews.llvm.org/D79111