[KnownFPClass] Offer other types of dynamic Denormal Modes

Currently, `DenormalMode` can represent the following output/input denormal modes: `ieee`, `preservesign`, `positivezero`, and `dynamic`.

The `dynamic` mode assumes that all three denormal modes are possible. However, I would imagine that some hardware floating point units may only support two of the three denormal modes, (e.g. only `ieee` and `preservesign`), so we can make a few more deductions if the denormal mode is either `ieee` or `preservesign` but not `positivezero`.

One way we can represent things is with a bitmask, where `bit 0 = ieee`, `bit 1 = preservesign`, `bit 2 = positivezero`. All bits set would be the current `dynamic` mode, and a denormal mode that is either `ieee` or `preservesign` would be `0b011`. All bits cleared would refer to the current `invalid` state.

Another thing we could also add for the input denormal mode is `maybepreservesign` and `alwayspreservesign`, with the latter being a guarantee that input denormals always flush to zero.

Pros:
- Slightly stronger deductions with regards to output/input denormals.

Cons:
- More maintenance work, as the number of possible output/input modes goes from `4 * 4 = 16` to `7 * 7 = 49` (Assuming the output/input denormal mode cannot be `invalid`).

***

Also, currently, we would have to do a lot of work anyways if we were to add a new denormal mode (for example, one that flushes both subnormals and negative zero to positive zero).

Start by locating the KnownFPClass handling of DenormalMode and reviewing how the current ieee, preservesign, positivezero, and dynamic states are represented. Resolve the proposed representation and input-mode additions before implementing them; done means the supported denormal combinations and resulting deductions are represented consistently.

Written by the indexing model from the issue text.