[llvm] specify NaN behavior more precisely (PR #66579)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

> A bunch of changes happened since it got approved though. I'm not familiar with how LLVM handles that situation.

If you think the changes are significant enough to require another review, please point out the specific parts.  Otherwise, we can consider the patch ready.

> Also I don't have that button. :)

>From the developer policy: "If you do not have commit access, please let people know during the review and someone should commit it on your behalf."

https://github.com/llvm/llvm-project/pull/66579