[compiler] Flatten scopes based on fallthrough, not scope range

Once we create scopes, we should prefer to use the block structure to identify active scope ranges rather than the scope range. They _should_ always be in sync, but ultimately the block structure determine the active range (ie the id of the 'scope' terminal and the terminal's fallthrough block).

ghstack-source-id: 730b6d1cfaf0eb689d71057c78a48045ac4fb11c
Pull Request resolved: facebook#30398