Fall back to single-lane SLP before falling back to no SLP

Fall back to single-lane SLP before falling back to no SLP

The following changes the fallback to disable SLP when any of the
discovered SLP instances failed to pass vectorization checking into
a fallback that emulates what no SLP would do with SLP - force
single-lane discovery for all instances.

The patch does not remove the final fallback to disable SLP but it
reduces the fallout from failing vectorization when any non-SLP
stmt survives analysis.

	* tree-vectorizer.h (vect_analyze_slp): Add force_single_lane
	parameter.
	* tree-vect-slp.cc (vect_analyze_slp_instance): Remove
	defaulting of force_single_lane.
	(vect_build_slp_instance): Likewise.  Pass down appropriate
	force_single_lane.
	(vect_analyze_slp): Add force_sigle_lane parameter and pass
	it down appropriately.
	(vect_slp_analyze_bb_1): Always do multi-lane SLP.
	* tree-vect-loop.cc (vect_analyze_loop_2): Track two SLP
	modes and adjust accordingly.
	(vect_analyze_loop_1): Save the SLP mode when unrolling.

	* gcc.dg/vect/vect-outer-slp-1.c: Adjust.