Moody’s Investors Service on Monday downgraded Riviera Holdings Corp.’s debt ratings and said further downgrades are possible for the Las Vegas hotel-casino company. Riviera is considered to be in better financial shape after its emergence from bankruptcy last year — a deal engineered by veteran hotelier Barry Sternlicht. Moody’s, however, is worried about continued losses at the aging 2,075-room Riviera hotel-casino on the Las Vegas Strip as well as the company’s debt load of about $73 million.