Framework for non-local branches in ControlFlowVisitor (#1933)
Framework for non-local branches in ControlFlowVisitor (#1933)

- add support for tracking named 'global' flow paths between different
  parts of the IR, useful for tracking data flow over things like
  exit/return or calls or state transitions.
hanw/p4c,p4lang/p4c,p4lang/p4c,p4lang/p4c,mbudiu-vmw/p4c-clone,mbudiu-vmw/p4c-clone,mbudiu-vmw/p4c-clone,p4lang/p4c,mbudiu-vmw/p4c-clone,hanw/p4c,hanw/p4c,p4lang/p4c,hanw/p4c