MD5_SHIFT_13
FF(c, d, a, b, x_2, MD5_SHIFT_13, MD5_CONST_e(2)); /* 3 */
FF(c, d, a, b, x_6, MD5_SHIFT_13, MD5_CONST_e(6)); /* 7 */
FF(c, d, a, b, x_10, MD5_SHIFT_13, MD5_CONST_e(10)); /* 11 */
FF(c, d, a, b, x_14, MD5_SHIFT_13, MD5_CONST_e(14)); /* 15 */