More consistently split SLP groups

This appropriately makes matches all true after successful SLP discovery
to reliably succeed splitting.  We were picking up an eventual all
false built-up from scalars state in some cases.

2020-10-12  Richard Biener  <[email protected]>

	* tree-vect-slp.c (vect_analyze_slp_instance): Set matches to true
	after successful discovery but forced split.

Author: rguenth

gcc/tree-vect-slp.c

@@ -2283,7 +2283,7 @@ vect_analyze_slp_instance (vec_info *vinfo,
 	    dump_printf_loc (MSG_NOTE, vect_location,
 			     "SLP discovery succeeded but node needs "
 			     "splitting\n");
-	  matches[0] = true;
+	  memset (matches, true, group_size);
 	  matches[group_size / const_max_nunits * const_max_nunits] = false;
 	  vect_free_slp_tree (node);
 	}