[sgen-bridge] Also separate old bridge into pre- and post-callback.
[sgen-bridge] Set bridge_processing_in_progress in sgen-bridge.c.
[sgen-bridge] Move bridge debug option handling to sgen-bridge.c.
[sgen-bridge] Small refactoring.
[sgen-bridge] Allow second bridge implementation to run alongside the active one.
[sgen-bridge] Compare SCCs between bridge implementations.
[sgen-bridge] Compare xrefs between bridge implementations, too.