[LegalizeTypes][VP] Use LoVT/HiVT when splitting VP operations in SplitVecRes_UnaryOp.

The VP path was using the split source VTs instead of the split
destination VTs. This may not be a problem today because the VP
nodes going through this have the same source and dest VTs.
It will be a problem when we start using this function for legalizing
VP cast operations.