[clang] [llvm] Reapply "InstCombine: Introduce SimplifyDemandedUseFPClass"" (PR #74056)

[Jay Foad via cfe-commits]

jayfoad wrote:

> The referenced issue violates the spec for finite-only math only by
> using a return value for a constant infinity.

You mean this issue? https://github.com/llvm/llvm-project/commit/5a36904c515b#commitcomment-129847939

Can you explain how your patch "broke" it? If you return infinity from a function marked with `ninf`, I would expect your patch to have no effect, because `DemandedMask & Known.KnownFPClasses` will be empty so `getFPClassConstant` will return `nullptr`.

https://github.com/llvm/llvm-project/pull/74056