[RISCV] Peak through BITCAST in isUsedByReturnOnly

A follow up to 47b1f836,
This covers some more cases on soft-float ABIs with the f/d extension.
I left the TODO in as there is still the case where we need to handle rv32i without f/d, but that looks like it requires changes to SelectionDAGLegalize::ExpandFPLibCall

Reviewed By: asb, reames

Differential Revision: https://reviews.llvm.org/D142911