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

[Craig Topper via llvm-commits]

topperc wrote:

> > Can be fixed up elsewhere (i.e during lowering) but think generally its indicative of the somewhat builtin "signedness" of floats and thus how si is a better match.
> 
> I think this says a lot more about the x86 architecture than about floating point numbers :) Pre-avx512 is well known to have very weird holes in the simd instruction set -- you could equally say that integers have an inherent preference for signed comparison because x86 can't compare packed unsigned numbers...

I think there's an asymmetry somewhere in PowerPC too. But its for the other direction fptosi/fptoui. See this comment from `DAGTypeLegalizer::PromoteIntRes_FP_TO_XINT`

```
  // If we're promoting a UINT to a larger size and the larger FP_TO_UINT is     
  // not Legal, check to see if we can use FP_TO_SINT instead.  (If both UINT    
  // and SINT conversions are Custom, there is no way to tell which is           
  // preferable. We choose SINT because that's the right thing on PPC.)
```

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