By Sam Nussey TOKYO (Reuters) -SoftBank Group Corp said on Wednesday it would book a gain of $34.1 billion by cutting its stake in Alibaba Group Holding, as the investment behemoth looks to shore up its cash reserve to weather the market downturn. The Japanese company will reduce its stake in Alibaba to 14.6% from 23.7% by settling prepaid forward contracts. Alibaba’s U.S.-listed shares were down 1.3% in early trade. SoftBank booked a $50 billion loss at its Vision Fund investment arm in the first half of the year as its tech bets soured, with Chief Executive Masayoshi Son on Monday pledging t…