tree-optimization/116810 - out-of-bound access to matches[]

The following makes sure to apply forced splitting of groups for
firced single-lane SLP only when the group being analyzed has more
than one lane.  This avoids an out-of-bound access to matches[].

	PR tree-optimization/116810
	* tree-vect-slp.cc (vect_build_slp_instance): Onlu force
	splitting for group_size > 1.