[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.
> 
> [dtcxzyw/llvm-opt-benchmark#248 (comment)](https://github.com/dtcxzyw/llvm-opt-benchmark/pull/248#discussion_r1515850373) But this patch transforms `add + sitofp` into `uitofp + fadd` :(

Ah I see, thought you meant the other way around.

Okay, think what needs to change is need to be able to handle `uitofp` as `sitofp`
in `foldFBinOpOfIntCasts`. Ill make a patch.

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