[llvm] Avoid maxnum(sNaN, x) optimizations / folds (PR #170181)

[Phoebe Wang via llvm-commits]

https://github.com/phoebewang approved this pull request.

I'm good with the change assuming sNaN is rare in reality. I still want it back once we made consensus in LangRef.

https://github.com/llvm/llvm-project/pull/170181