Clean up generator live locals analysis

Instead of using a bespoke dataflow analysis, `MaybeRequiresStorage`,
for computing locals that need to be stored across yield points and that
have conflicting storage, use a combination of simple, generally
applicable dataflow analyses. In this case, the formula for locals
that are live at a yield point is:

    live_across_yield := (live & init) | (!movable & borrowed)

and the formula for locals that require storage (and thus may conflict
with others) at a given point is:

    requires_storage := init | borrowed

`init` is `MaybeInitializedLocals`, a direct equivalent of
`MaybeInitializedPlaces` that works only on whole `Local`s. `borrowed`
and `live` are the pre-existing `MaybeBorrowedLocals` and
`MaybeLiveLocals` analyses respectively.

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