[llvm] [KnownFPClass] Refactor sNaN handling for KnownFPClass (PR #216086)

[via llvm-commits]

ZERICO2005 wrote:

@arsenm could I get a review for this? This PR addresses your comment here https://github.com/llvm/llvm-project/pull/215159#pullrequestreview-4898780491. 

I have a few follow-up PRs blocked on this since I renamed `propagateNaN` to `propagateNonNaN` in this PR, and I do not want to introduce more uses of the old name of `propagateNaN`.


https://github.com/llvm/llvm-project/pull/216086