[InstCombine] fold fcmp with lossy casted constant

This is noted as a missing clang warning in #54222
(and we should still make that enhancement).

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.