Removing unnecessary check and shortening the code.

webform-form.tpl.php

<?php

foreach ($form['submitted'] as &$value) {
  $types = array('textfield', 'webform_email', 'textarea');

  if (isset($value['#type']) && in_array($value['#type'], $types)) {
    $value['#attributes']['placeholder'] = $value['#title'];
  }
}