NEW YORK, May 13 : A stunning run-up in shares of semiconductor companies has helped drive the U.S. stock rally, but the eye-popping gains are sparking concerns about an overheated market and prompting some investors to prepare for a pullback. The latest wave of artificial-intelligence market enthusiasm is buoying semiconductors broadly, after bellwether Nvidia symbolized the AI trade over most of the bull stock market that began in late 2022. That fervor spread throughout the industry this