[llvm] [KnownFPClass] Refactor NaN sign encoding for KnownFPClass (PR #220606)

[Eric Ross via llvm-commits]

ZERICO2005 wrote:

Rebased onto `main` due to https://github.com/llvm/llvm-project/pull/225575, since using `Known.setSignBit(std::nullopt)` is no longer needed for `applyInputDenormalMode`/`applyOutputDenormalMode`.

https://github.com/llvm/llvm-project/pull/220606