ir_visit_tree: Make sure we visit dereference targets, too.

Found this with the local dead code pass, which never saw variable
dereferences occurring.

ir_visit_tree.cpp

@@ -136,6 +136,7 @@ ir_tree_visitor::visit(ir_dereference *ir)
    if (ir->mode == ir_dereference::ir_reference_array) {
       ir->selector.array_index->accept(this);
    }
+   ir->var->accept(this);
 }
 
 void