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

[Nikita Popov via llvm-commits]

nikic wrote:

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

Why is it a better canonicalization? We prefer unsigned over signed operations for everything else (zext, lshr, udiv, icmp u), so this would be a very unusual.

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