compiler: Represent pruned scopes instead of inlining

There are a few places where we want to check whether a value actually got memoized, and we currently have to infer this based on values that "should" have a scope and whether a corresponding scope actually exists. This PR adds a new ReactiveStatement variant to model a reactive scope block that was pruned for some reason, and updates all the passes that prune scopes to instead produce this new variant.

ghstack-source-id: aea6dab469acb1f20058b85cb6f9aafab5d167cd
Pull Request resolved: https://github.com/facebook/react/pull/29781