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

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

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

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