[haifa-sched] PR rtl-optimization/68236: Exit early from autoprefetcher lookahead if not in haifa sched

	PR rtl-optimization/68236
	* haifa-sched.c (autopref_multipass_dfa_lookahead_guard): Return 0
	if insn_queue doesn't exist.
	(haifa_sched_finish): Reset insn_queue to NULL.

From-SVN: r230088