[llvm] [InstCombine] Preserve the sign bit of NaN in `SimplifyDemandedUseFPClass` (PR #137287)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Looks fine, but I don't think there is enough coverage for the new FMF handling? ninf is untested.

I think it is, just not standalone without nnan 

https://github.com/llvm/llvm-project/pull/137287