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

[Ralf Jung via llvm-commits]

RalfJung wrote:

> 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.

I find it hard to judge whether the reviewers would consider the changes significant enough. I am surprised that as an outsider to the project, I am expected to make such a call.

I was about to produce a summary of the changes since the approval, but thankfully @jyknight  was faster. :)

>  can you rewrite the commit description (by editing the title&first comment) to accurately describe the current situation?

I've updated the PR description. The PR title still seems fine?

Should I squash the commits?

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