By Alexei Oreskovic and Ross Kerber SAN FRANCISCO/BOSTON (Reuters) – Alibaba Group Holding Ltd’s upcoming public offering will give Yahoo Inc billions of dollars for a potential shopping spree that investors hope will jumpstart its comeback. But with numerous holes to fill and soaring tech company valuations, Yahoo will have to make tough choices with any acquisitions. Yahoo is expected to sell about $9.5 billion in stock of the Chinese e-commerce company. After giving at least half back to investors and paying taxes, Yahoos cash stockpile, currently $4.2 billion, may swell by $3.1 billion. …