[llvm] [KnownFPClass] Add output/input denormal helpers (PR #225575)

[Eric Ross via llvm-commits]

ZERICO2005 wrote:

Landing this with tgymnich's approval since several pending PRs depend on these helpers. @arsenm, happy to address any feedback on the API in a follow-up.

https://github.com/llvm/llvm-project/pull/225575