DevTools: Audit places where we change tags or disconnect alternates in React

Cases like https://github.com/bvaughn/react-devtools-experimental/issues/197 (where a dehydrated Suspense node turns into a regular one) produce confusing failures because we expect Fiber alternates to be "for life", whereas in practice they can actually get disconnected by React in some cases. (Search for "Disconnect" in ReactFiberBeginWork.) Additionally, I think changing `tag` can also produce confusing failures if it changes from a value that was filtered out, to a value that is not filtered out. We need to be more proactive about handling these cases when we make such changes to React, and we need to look at existing cases where this happens and whether we can handle them. --- Originally reported by @gaearon via https://github.com/bvaughn/react-devtools-experimental/issues/198