[RISCV] Handle fcopysign(f32, f64) and fcopysign(f64, f32)

Summary: Adds tablegen patterns to explicitly handle fcopysign where the
magnitude and sign arguments have different types, due to the sign value casts
being removed the by DAGCombiner. Support for RV32IF follows in a separate
commit. Adds tests for all relevant scenarios except RV32IF.

Reviewers: lenary
Reviewed By: lenary
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D70678