[llvm] [LangRef] make consequences of NaN rules for pow(i) more explicit (PR #170177)

[Joshua Cranmer via llvm-commits]

https://github.com/jcranmer-intel approved this pull request.

I keep on going back and forth over the amount of detail that's useful here, namely what you need to do if you want sNaN to not act as qNaN. At the end of the day, though, I think that's best handled with a distinct document on floating-point semantics that's already in my queue to write.

My biggest concern is that the eventual move away from constrained intrinsics will require modification of this wording, but it's probably best to deal with that when we make that move rather than trying to do the wording for that scenario right now.

https://github.com/llvm/llvm-project/pull/170177