[Rust] Add LowLevelILSSARegisterKind::id helper

[emesare]

The partial register id is the "main" id so we return that, whereas the full register id is not.

@bdash did you have any more thoughts surrounding the usage of this API?

[bdash]

I'm not sure about this API. It does not seem obvious to a client what id will return for a partial register. Maybe a doc comment fixes that, or perhaps there's a clearer name.

I'm not even 100% sure whether get_ssa_register_uses and friends are doing the right thing with how they handle partial registers at the moment.