[llvm] [DAG] Fixing the non-optimal code with the following: `select i1 %0, float 1.0, float 0.0`. (PR #107732)

[via llvm-commits]

c8ef wrote:

> > https://github.com/llvm/llvm-project/blob/caebb4562ce634a22f7b13480b19cffc2a6a6730/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp#L17953-L17959
> > It seems that when applying this patch, we are switching back and forth between `select` and `uint_to_fp`.
> > ```
> >   // fold (uint_to_fp (setcc x, y, cc)) -> (select (setcc x, y, cc), 1.0, 0.0)
> >   if (N0.getOpcode() == ISD::SETCC && !VT.isVector() &&
> >       (!LegalOperations || TLI.isOperationLegalOrCustom(ISD::ConstantFP, VT))) {
> >     SDLoc DL(N);
> >     return DAG.getSelect(DL, VT, N0, DAG.getConstantFP(1.0, DL, VT),
> >                          DAG.getConstantFP(0.0, DL, VT));
> >   }
> > ```
> 
> How about doing it in `lowerSELECT`? It looks like RISCV does it in this way without infinite loop: https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/RISCV/RISCVISelLowering.cpp#L8002

Done. The CI is currently green, but I'm unsure if the affected test files exhibit regression.

https://github.com/llvm/llvm-project/pull/107732