class User extends CI_Controller {

    public function __construct() {
        parent::__construct();
        $this->load->model('User_model');
    }

    public function index() {
        $this->load->view('registration/index');
    }

    public function login() {
        $this->load->view('registration/index');
    }

<li class="nav-item">
    <a class="nav-link" href="<?php echo site_url("user/login")?>">Login</a>
</li>
<li class="nav-item">
    <a class="nav-link" href="<?php echo site_url("user")?>">Register</a>
</li>