[LegalizeTypes][NFC] Combine ExpandIntOp_{S,U}INT_TO_FP to ExpandIntOp_XINT_TO_FP

As with D157399 we can reduce duplication by doing this. Unlike that
patch, I'm posting the refactoring before the functional changes I want
to make here.

Differential Revision: https://reviews.llvm.org/D157403