[bitcoin/bitcoin] p2p: Clarify control flow in ProcessMessage (#13946)

I’m not sure this is **much** better with the early returns. though, this does untangle the case and makes it easier to split up the function and move to dispatching based on say, a hash table, in the future, so it’s a step in the right direction.

utACK fa6c3dea420b6c50c164ccc34f4e9e8a7d9a8022