title: How do I know whether a UITableViewCell currently exists?
body: <p>Given the <code>indexPath</code> of a UITableViewCell, I want to know whether the cell currently exists. </p>
  
  <p>I know how to check if it's on the screen (use <code>tableView.indexPathsForVisibleRows</code>). However, I also want to know if it's off the screen but already created (assuming the user has scrolled but it hasn't quite entered the screen are yet).</p>
  
  <p>How do I do this?</p>