I asked a more general (non-WordPress specific) question about polyfilling on StackOverflow (I strongly recommend reading through the ensuing comments for context) which has lead me to the following answer:
add_action( 'login_enqueue_scripts', 'wpse_215432_oldbrowsersupport');
function wpse_215432_oldbrowsersupport() {
wp_register_script( 'ie_html5shiv', get_stylesheet_directory_uri().'/js/html5shiv.min.js', __FILE__, false, '3.7.3' );
wp_enqueue_script( 'ie_html5shiv');
wp_script_add_data( 'ie_html5shiv', 'conditional', 'lt IE 9' );
//Note: Placeholder script (https://github.com/mathiasbynens/jquery-placeholder) covered by a plugin https://wordpress.org/plugins/html5-placeholder-polyfill/
wp_register_script( 'ie_selectivizr', get_stylesheet_directory_uri().'/js/selectivizr.js', __FILE__, false, '1.0.3' );
wp_enqueue_script( 'ie_selectivizr');
wp_script_add_data( 'ie_selectivizr', 'conditional', 'lt IE 9' );
}