/**
 * Implements hook_element_info_alter().
 */
function module_element_info_alter(array &$types) {
  // Add placeholders to password_confirm element.
  $types['password_confirm']['#process'][] = "_module_add_pass_confirm_placeholders";
}

function _module_add_pass_confirm_placeholders(&$element, FormStateInterface $form_state, &$complete_form) {
  $element['pass1']['#placeholder'] = t('Password');
  $element['pass2']['#placeholder'] = t('Confirm password');

  return $element;
}