[llvm] [RISCV][ISel] Fold FSGNJX idioms (PR #100718)

[Philip Reames via llvm-commits]

preames wrote:

> > Any reason this can't be done by e.g. instcombine?
> 
> What IR could InstCombine generate?

We have a llvm.copysign, so wouldn't this just be (copysign X, Y)?  Or is there some corner case here I'm missing?

https://github.com/llvm/llvm-project/pull/100718