[llvm] ValueTracking: Treat poison more aggressively in computeKnownFPClass (PR #87990)

[Joshua Cranmer via llvm-commits]

https://github.com/jcranmer-intel commented:

Can you add a test for `freeze poison` as well? From looking through the code, it should be the case that it does the right thing already, but having a test makes me feel better that we won't accidentally break it in future changes.

https://github.com/llvm/llvm-project/pull/87990