Alibaba Group Holding Ltd may be making its debut on Wall Street later this summer, but it’s certainly no stranger to the investors of Silicon Valley. In the past 18 months, the Chinese e-commerce giant has burrowed its way into the Valley’s exclusive investment circles, snapping up sizeable stakes and board seats in fast-growing startups that could provide strategic advantages for when, not if, the company challenges Amazon.com Inc or eBay Inc on their home turf. A U.S.