ValueTracking: Identify implied fp classes by general fcmp (#66505)

Previously we could recognize exact class tests performed by
an fcmp with special values (0s, infs and smallest normal).
Expand this to recognize the implied classes by a compare with a general
constant. e.g. fcmp ogt x, 1 implies positive and non-0.
    
The API should be better merged with fcmpToClassTest but that
made the diff way bigger, will try to do that in a future
patch.