Try like this:
$data = array('current_login' => date('Y-m-d H:i:s'));
$this->db->set('last_login', 'current_login', false);
$this->db->where('id', 'some_id');
$this->db->update('login_table', $data);
Pay particular attention to the set() call's 3rd parameter. false prevents CodeIgniter from quoting the 2nd parameter -- this allows the value to be treated as a table column and not a string value. For any data that doesn't need to special treatment, you can lump all of those declarations into the $data array.
The query generated by above code:
UPDATE `login_table`
SET last_login = current_login, `current_login` = '2018-01-18 15:24:13'
WHERE `id` = 'some_id'