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

[Joshua Cranmer via cfe-commits]

https://github.com/jcranmer-intel commented:

There's probably some useful discussion to be had about how aggressive `-ffinite-math-only` at the clang level should be wrt lowering to `nnan`/`ninf` in the IR.

It may be worth deferring the ReturnInst changes (but landing everything else) until that discussion is had.

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