WASHINGTON—SoftBank Group Corp’s Vision Fund exited its bet on General Motors (GM) self-driving car subsidiary Cruise as the auto giant upped the ante, investing another $3.45 billion in the loss-making unit. SoftBank’s departure, which comes as the Japanese conglomerate struggles with debts, prompted questions from sector watchers about whether Cruise is ready to generate meaningful revenue for the time being, and how deep GM might have to dig to fund development. GM said on Friday that it has agreed to pay $2.1 billion to buy the SoftBank Vision Fund stake in Cruise, and to make a separate $1.35 billion investment in the unit that SoftBank had committed to make in 2018. Combined, the transactions will bring GM’s stake in Cruise to 80 percent, GM said. Other remaining shareholders in Cruise include Microsoft, Walmart, and Honda Motor Co. In 2018, SoftBank invested $900 million in Cruise and said it would invest …