Clean up logic around live locals in generator analysis

[ecstatic-morse]

Resolves #69902. Requires #71893.

I've found it difficult to make changes in the logic around live locals in generator/transform.rs. It uses a custom dataflow analysis, MaybeRequiresStorage, that AFAICT computes whether a local is either initialized or borrowed. That analysis is using before effects, which we should try to avoid if possible because they are harder to reason about than ones only using the unprefixed effects. @pnkfelix has suggested removing "before" effects entirely to simplify the dataflow framework, which I might pursue someday.

This PR replaces MaybeRequiresStorage with a combination of the existing MaybeBorrowedLocals and a new MaybeInitializedLocals. MaybeInitializedLocals is just MaybeInitializedPlaces with a coarser resolution: it works on whole locals instead of move paths. As a result, I was able to simplify the logic in compute_storage_conflicts and locals_live_across_suspend_points.

This is not exactly equivalent to the old logic; some generators are now smaller than before. I believe this was because the old logic was too conservative, but I'm not as familiar with the constraints as the original implementers were, so I could be wrong. For example, I don't see a reason the size of the mixed_sizes future couldn't be 5K. It went from 7K to 6K in this PR.

r? @jonas-schievink @tmandry