[InstCombine] fold fcmp with lossy casted constant (2nd try)

This is a retry of 9397bdc6 - that was reverted until
we had a clang warning in place to alert users about a
possible mistake in source. The warning was added with
ab982eac.

This is noted as a missing clang warning in #54222,
but it is also a missing optimization opportunity.

Alive2 proofs:
https://alive2.llvm.org/ce/z/Q8drDq
https://alive2.llvm.org/ce/z/pE6LRt

I don't see a single conversion for all predicates
using "getFCmpCode" logic, so other predicates are
left as a TODO item.