Add double reduction support for SLP vectorization

The following makes double reduction vectorization work when
using (single-lane) SLP vectorization.

	* tree-vect-loop.cc (vect_analyze_scalar_cycles_1): Queue
	double reductions in LOOP_VINFO_REDUCTIONS.
	(vect_create_epilog_for_reduction): Remove asserts disabling
	SLP for double reductions.
	(vectorizable_reduction): Analyze SLP double reductions
	only once and start off the correct places.
	* tree-vect-slp.cc (vect_get_and_check_slp_defs): Allow
	vect_double_reduction_def.
	(vect_build_slp_tree_2): Fix condition for the ignored
	reduction initial values.
	* tree-vect-stmts.cc (vect_analyze_stmt): Allow
	vect_double_reduction_def.