* tree-ssa-threadbackward.c (fsm_find_thread_path): Make GC
	vectors heap vectors.  Clean up comments.
	Make visited_bbs a reference.
	(profitable_jump_thread_path): Make GC
	vectors heap vectors.  Clean up comments.
	Misc cleanups.
	(convert_and_register_jump_thread_path): Make GC vectors heap
	vectors.
	(check_subpath_and_update_thread_path): Same.  Clean up comments.
	Make visited_bbs a reference.
	(handle_phi): Abstract common code to to
	register_jump_thread_path_if_profitable.
	Rename VAR_BB to DEF_BB.
	Update comments.
	Make GC vectors heap vectors.
	Make visited_bbs a reference.
	(handle_assignment): Same.
	(register_jump_thread_path_if_profitable): New.
	(fsm_find_control_statement_thread_paths): Rename VAR_BB to
	DEF_BB.
	Make GC	vectors heap vectors.  Clean up comments.
	Make visited_bbs a reference.
	(find_jump_threads_backwards): Make visited_bbs live in the stack.
	* tree-ssa-threadupdate.c (delete_jump_thread_path): Fix typo in
	comment.

From-SVN: r251998