Clear DR_GROUP_NEXT_ELEMENT upon group dissolving

I've tried to sanitize DR_GROUP_NEXT_ELEMENT accesses but there are too
many so the following instead makes sure DR_GROUP_NEXT_ELEMENT is never
non-NULL for !STMT_VINFO_GROUPED_ACCESS.

	* tree-vect-data-refs.cc (vect_analyze_data_ref_access): When
	cancelling a DR group also clear DR_GROUP_NEXT_ELEMENT.