[LegalizeTypes][RISCV] Soften FCOPYSIGN operand

Summary: Adds support for softening FCOPYSIGN operands.
Adds RISC-V tests that exercise the new softening code.

Reviewers: asb, lenary, efriedma
Reviewed By: efriedma
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D70679