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

[Nikita Popov via cfe-commits]

nikic wrote:

I don't think we really have a strong motivation for this change beyond "having a canonical form is usually good", so if it's causing issues for some targets, then probably just not doing it is fine. But if we do want to keep it, then yeah, this would be a reasonable motivation for adding the nneg flag.

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