[KnownFPClass] Add `KnownFPClass::frem` [NFC] (#218949)
Follow up to https://github.com/llvm/llvm-project/pull/218726
Moves `frem` deduction logic from `Analysis/ValueTracking.cpp` and
`CodeGen/GlobalISel/GISelValueTracking.cpp` to `KnownFPClass::frem`.
