[llvm] [KnownFPClass] Improve known class deductions for exp/exp2/exp10 (PR #217526)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

LGTM with a note that this is making an assumption about this only handling the formats with IR types (preferably with an example where it breaks) 

https://github.com/llvm/llvm-project/pull/217526