* basic-block.h (basic_block): Add new field 'flags'.
	(BB_REACHABLE): Define.
	(expunge_block): Declare.
	* flow.c (ENTRY_BLOCK_PTR): Initialize field 'flags'.
	(EXIT_BLOCK_PTR): Ditto.
	(expunge_block): Remove static declaration.
	(cleanup_cfg): Clear bb->aux on every basic block.
	(find_unreachable_blocks): Set BB_REACHABLE bit in bb->flags when
	computing reachability.
	(delete_unreachable_blocks): Delete block b if b->flags has
	BB_REACHABLE unset.

From-SVN: r45068