[RISCV] Add support for call returns to RISCVSExtWRemoval.

The implementation is inspired by code from PPCInstrInfo.

We look for a copy from X10(A0) preceded by an ADJCALLSTACKUP and
a PseudoCall. We use the PseudoCall to look up the IR function being
called to find it's return attributes.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D139808