[llvm] [RISCV][GISel] Support f32/f64 ldexp. (PR #117941)

[Sam Elliott via llvm-commits]

https://github.com/lenary approved this pull request.

LGTM. I'm happy that we should be doing the same thing for ldexp as we do for `__powi*` (given the signatures are the same). 

I do have a comment on the PR this is stacked on, which does block landing this, sorry.

https://github.com/llvm/llvm-project/pull/117941