[llvm] [KnownFPClass] Add KnownFPClass deductions for `pow` (PR #215592)

[via llvm-commits]

ZERICO2005 wrote:

This PR https://github.com/llvm/llvm-project/pull/216086 needs to be merged before this one (due to the renaming of NaN propagation helpers).

https://github.com/llvm/llvm-project/pull/215592