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

[via llvm-commits]

goldsteinn wrote:

My thinking was `sitofp` is a better canonicalization, but either way we should probably pick one.

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