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

[via llvm-commits]

goldsteinn wrote:

> Regression:
> 
> ```
> define float @src(i32 %shr.i) {
>   %and.i = and i32 %shr.i, 32767
>   %sub = uitofp i32 %and.i to float
>   %add = fadd float %sub, -16383.0
>   ret float %add
> }
> 
> define float @tgt(i32 %shr.i) {
>   %and.i = and i32 %shr.i, 32767
>   %addconv = add nsw i32 %and.i, -16383
>   %sub = sitofp i32 %addconv to float
>   ret float %sub
> }
> ```
> 
> Alive2: https://alive2.llvm.org/ce/z/W6Ik5C

Is that a regression?
Seems like the `fadd` -> `add` is desirable.
Likewise we don't really know return op sign in either.



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