re PR tree-optimization/71403 (wrong code (segfault) at -O3 on x86_64-linux-gnu)

re PR tree-optimization/71403 (wrong code (segfault) at -O3 on x86_64-linux-gnu)

	PR tree-optimization/71403
	* tree-ssa-threadbackward.c
	(convert_and_register_jump_thread_path): No longer accept reference
	to path.  Do not pop items off the path anymore.
	(fsm_find_control_statement_thread_paths): Do not allow threading
	to a deeper loop nest.  Pop the last item off the path here rather
	than in convert_and_register_jump_thread_path.

	PR tree-optimization/71403
	* c-c++-common/ubsan/pr71403-1.c: New test.
	* c-c++-common/ubsan/pr71403-2.c: New test.
	* c-c++-common/ubsan/pr71403-3.c: New test.

From-SVN: r237403