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

This reverts commit 96a0d714.

Avoid assert with dynamic denormal-fp-math We don't recognize compares
with 0 as an exact class test if we don't know the denormal mode. We could
try to do better here, but it's probably not worth it.

Fixes asserts reported after 1adce7d8e47e2438f99f91607760b825e5e3cc37