Fix iteration over loads in SLP optimize

I've made a typo when refactoring the iteration over all loads in
the SLP graph.  Fixed.

2020-10-28  Richard Biener  <rguenther@suse.de>

	* tree-vect-slp.c (vect_optimize_slp): Fix iteration over
	all loads.

gcc/tree-vect-slp.c

@@ -3043,7 +3043,7 @@ vect_optimize_slp (vec_info *vinfo)
   /* Now elide load permutations that are not necessary.  */
   for (i = 0; i < leafs.length (); ++i)
     {
-      node = vertices[i];
+      node = vertices[leafs[i]];
       if (!SLP_TREE_LOAD_PERMUTATION (node).exists ())
 	continue;