[llvm] [InstCombine] Add folds for `(fp_binop ({s|u}itofp x), ({s|u}itofp y))` (PR #82555)

[via llvm-commits]

goldsteinn wrote:

> can you also test/prove the binop (sitofp), (uitofp) cases

The `binop (sitofp), (uitofp)` as we handle them are a subset of the existing proofs.
We only handle different signed cases if we convert one to the either (i.e proof one of
the ops is non-neg: `(uitofp nneg x)` == `(sitofp nneg x)`).

https://github.com/llvm/llvm-project/pull/82555