tree-optimization/122722 - better SLP reduction group discovery

tree-optimization/122722 - better SLP reduction group discovery

The following improves the all-or-nothing discovery of reduction
groups to consider sub-groups by trying toplevel "matches" candidates
for this.  For simplicity and to limit compile-time failed sub-group
matches are not decomposed further, only the originally failed part
is tried again to discover more sub-groups.  Any remaining fails
get picked up by the current single-reduction handling.

	PR tree-optimization/122722
	* tree-vect-slp.cc (vect_analyze_slp_reductions): New
	function, split out from vect_analyze_slp.  Try SLP
	sub-groups.
	(vect_analyze_slp_reduction_group): New helper.

	* gcc.dg/vect/slp-reduc-14.c: New testcase.