[llvm] [InstCombine] Canonicalize `(uitofp x)` -> `(sitofp x)` if `x >= 0` (PR #82404)

[Nikita Popov via llvm-commits]

nikic wrote:

> Edit: The little test I added is an example.

Is there some inherent reason why that transform can only work with sitofp, rather than also uitofp with an s/signed/unsigned replacement everywhere? If there isn't, we should extend that transform instead.

https://github.com/llvm/llvm-project/pull/82404