Nikkei extends gains, SoftBank jumps on Alibaba earnings

* Bargain hunting after heavy fall last week underpins Nikkei * Market faces headwind from Ukraine, wobbly U.S. momentum shares, doubts on BOJ policy * SoftBank jumps after Alibaba reports strong earnings By Hideyuki Sano TOKYO, April 16 (Reuters) - Japan's Nikkei share average rose on Wednesday after a better performance on Wall Street overnight, with index heavyweight SoftBank soaring after a strong earnings report from its Chinese e-commerce affiliate Alibaba Group Holding Lt