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

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goldsteinn wrote:

See: https://github.com/llvm/llvm-project/pull/86141 which adds `nneg` flag to IR. Once that gets in I have patches for instcombine,cvp,sccp to do the transform.

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