nir/opt_loop: add loop peeling optimization

/**
 * This optimization tries to peel the first loop break.
 *
 * This optimization turns:
 *
 *     loop {
 *        do_work_1();
 *        if (cond) {
 *           break;
 *        } else {
 *        }
 *        do_work_2();
 *     }
 *
 * into:
 *
 *     do_work_1();
 *     if (cond) {
 *     } else {
 *        loop {
 *           do_work_2();
 *           do_work_1();
 *           if (cond) {
 *              break;
 *           } else {
 *           }
 *        }
 *     }
 *
 * nir_opt_dead_cf() can later remove the outer IF statement, again.
 *
 */