ValueTracking: Fix bug with fcmp false to nan constant

If we had a comparison to a literal nan with a false predicate,
we were incorrectly treating it as an unordered compare. This was
correct for fcmp true, but not fcmp false. I noticed this in the
review for e44d3b3e but misdiagnosed
the reason. Also change the test for the fcmp true case to be more
useful, but it wasn't wrong previously.