[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.

[emesare]

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.

Yea it seems that it was never intended to be queried with a partial register, I fixed up the LowLevelILSSARegisterKind to make it so that partial register will not be able to be passed to any of those api's. I also added some more doc comments to hopefully try and explain what Full and Partial ssa register kinds are.

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