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

[via llvm-commits]

goldsteinn 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.

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