Assert that the given node in set_r_cur_block() has mode BB.

ir/ir/ircons.c

@@ -557,7 +557,8 @@ void set_cur_block(ir_node *target)
 
 void set_r_cur_block(ir_graph *irg, ir_node *target)
 {
-	assert(target == NULL || irg == get_irn_irg(target));
+	assert(target == NULL || get_irn_mode(target) == mode_BB);
+	assert(target == NULL || get_irn_irg(target)  == irg);
 	irg->current_block = target;
 }