U.S. mutual funds are placing bigger bets on privately held companies to get a head start finding the next IPO superstar, a strategy that has yielded some dramatic payoffs and flameouts. Fast-growing private companies such as Uber Technologies, Pinterest and India’s Flipkart Online Services have attracted billions of dollars in investment from U.S. mutual funds, marking an increase in overall pre-IPO betting over the past few years, analysts and fund managers said. Funds run by Boston-based Fidelity Investments and Baltimore’s T.