nav_menu_css_class

function alpha_menu_item_class( $classes, $item ) {
    $classes[] = 'list-inline-item';
    return $classes;
}
add_filter( 'nav_menu_css_class', 'alpha_menu_item_class', 10, 2 );

https://codex.wordpress.org/Plugin_API/Filter_Reference/nav_menu_css_class