[KnownFPClass] Fix fsub input denormal handling (#225605)
Fixes https://github.com/llvm/llvm-project/issues/225587
Refactors the output/input denormal handling for `KnownFPClass::fadd`
and `KnownFPClass::fsub`.
`fadd`/`fadd_self`/`fsub` handle the flushing of output/input denormals,
and will pass the flushed inputs to `fadd_impl` (which uses IEEE
subnormal handling).
`KnownFPClass::fma` remains unchanged (note that the `DenormalMode`
parameter was unused by `fadd_impl`).
