[llvm] [DAGCombiner] Simplifying `{si|ui}tofp` when only signbit is needed (PR #85445)

[Jay Foad via llvm-commits]

jayfoad wrote:

> > Do you really have to duplicate the same logic in `SimplifyMultipleUseDemandedBits` and `SimplifyDemandedBits`?
> 
> I can make it a helper. My feeling was its only 1-2 lines of code so not really needed.

It's more of a question about the design of `SimplifyMultipleUseDemandedBits` vs `SimplifyDemandedBits` - surely we shouldn't need to add the same cases to both of them? @RKSimon?

https://github.com/llvm/llvm-project/pull/85445