Drive-by refactor: use `OnceCell` for the reverse region SCC graph
During region inference, the reverse SCC region graph is sometimes computed lazily. This changes the implementation for that from using an `Option` to a `OnceCell` which clearly communicates the intention and simplifies the code somewhat.
There shouldn't be any performance impact, except that this pulls the computation of the reverse SCC graph slightly later than before, and so may avoid computing it in some instances.
Note that this changes a mutable reference into an immutable (interior mutable) one.
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