[PATCH][PR tree-optimization/67892] Use FSM threader to handle backedges

[PATCH][PR tree-optimization/67892] Use FSM threader to handle backedges

	PR tree-optimization/67892
	* tree-ssa-threadedge.c (simplify_controL_stmt_condition): Fix typo
	in comment.
	(thread_through_normal_block): If we have seen a backedge, then
	do nothing.  No longer call find_jump_threads_backwards here.
	(thread_across_edge): Use find_jump_threads_backwards to find
	jump threads if the old style threader was not successful.
	* tree-ssa-threadbackward.c (get_gimple_control_stmt): Use
	gsi_last_nondebug_bb.  Return NULL if the block does not end
	with a control statement.
	(find_jump_threads_backwards): Setup code moved here from
	tree-ssa-threadedge.c::thread_through_normal_block.  Accept
	single edge argument instead of name & block.
	* tree-ssa-threadbackward.h (find_jump_threads_backwards): Update
	prototype.

        PR tree-optimization/67892
	* gcc.dg/tree-ssa/pr21417: Update expected output.
	* gcc.dg/tree-ssa/ssa-dom-thread-2b.c: Likewise.

From-SVN: r229538