title: tableView.register(nib) loses auto layout constraints
body: <p>in my <code>viewDidLoad</code> I register my nib:</p>
  
  <pre><code>tableView.register(UINib(nibName: "MyCell", bundle: nil), forCellReuseIdentifier: "MyCell")
  </code></pre>
  
  <p>which I fetch in my <code>cellForRowAtIndexPath</code>:</p>
  
  <pre><code>guard let cell = tableView.dequeueReusableCell(withIdentifier: "MyCell") as? MyCell else {
      return UITableViewCell()
  }
  // I do get here, but no constraints exist
  </code></pre>
  
  <p>In my <code>MyCell.xib</code> which is a <code>UITableViewCell</code>, I layout my cell content with auto layout; but when running the app, all the cells subviews stick in the upper left, no constraints exist.</p>
  
  <p>How can I achieve that the auto layout of <code>MyCell.xib</code> 'survives'?</p>