re PR tree-optimization/71550 (wrong code at -O3 on x86_64-linux-gnu)

	PR tree-optimization/71550
	PR tree-optimization/71403
	* tree-ssa-threadbackward.c: Include tree-vectorizer.h
	(profitable_jump_thread_path): Also return boolean indicating if
	the realized path will create an irreducible loop.
	Remove loop depth tests from 71403.
	(fsm_find_control_statement_thread_paths): Remove loop depth tests
	from 71403.  If threading will create an irreducible loop, then
	throw away loop iteration and related information.

	PR tree-optimization/71550
	PR tree-optimization/71403
	* gcc.c-torture/execute/pr71550.c: New test.

From-SVN: r240727