be: Replace edges_reroute() + kill_node() by exchange().

ir/be/amd64/amd64_finish.c

@@ -132,10 +132,8 @@ static void transform_sub_to_neg_add(ir_node *node,
 	arch_set_irn_register(add_res, out_reg);
 
 	/* exchange the add and the sub */
-	edges_reroute(node, add);
 	sched_replace(node, add);
-
-	kill_node(node);
+	exchange(node, add);
 }
 
 static ir_node *amd64_turn_back_am(ir_node *node)

ir/be/ia32/ia32_finish.c

@@ -169,16 +169,12 @@ carry:;
 		}
 	}
 
-	/* exchange the add and the sub */
-	edges_reroute(irn, res);
-	sched_replace(irn, res);
-
 	set_irn_mode(res, get_irn_mode(irn));
-
 	SET_IA32_ORIG_NODE(res, irn);
 
-	/* remove the old sub */
-	kill_node(irn);
+	/* exchange the add and the sub */
+	sched_replace(irn, res);
+	exchange(irn, res);
 }
 
 static inline int need_constraint_copy(ir_node *irn)