re PR tree-optimization/71335 (wrong code at -O2 and -O3 in 32-bit and 64-bit modes on x86_64-linux-gnu)

	PR tree-optimization/71335
	* tree-ssa-threadbackward.c (profitable_jump_thread_path): Filter out
	zero length paths here.
	(convert_and_register_jump_thread_path): Remove hacks related to
	duplicated blocks in the jump thread path.
	(fsm_find_control_statement_thread_paths): Avoid putting the same
	block on the thread path twice, but ensure the thread path is
	unchanged from the caller's point of view.

	PR tree-optimization/71335
	* gcc.c-torture/execute/pr71335.c: New test.

From-SVN: r237312