Global tech investor SoftBank posted a $7.7 billion profit in the third quarter, its largest in two years by far. Aiding results: a run-up in shares from an old investment in T-Mobile that long provided humdrum returns. The gains come over a decade after SoftBank bought U.S. carrier Sprint in 2013, intending to merge it with T-Mobile and create a giant mobile provider to compete head-to-head with AT&T and Verizon.