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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

SimplifyMultipleUseDemandedBits could be enough, that would need testing. Often SimplifyDemandedBits doesn't make fallback calls to SimplifyMultipleUseDemandedBits when it could.

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