By Makiko Yamazaki and Junko Fujita TOKYO (Reuters) – Toshiba Corp’s top shareholder said it had agreed to sell its stake to Bain Capital if the U.S. private equity firm launched a tender offer, potentially forcing the embattled conglomerate to revive talks for a private equity buyout. The agreement between Bain and Singapore-based Effissimo Capital Management, which owns about 9.9% of Toshiba, comes as shareholders last week voted down the Japanese industrial conglomerate’s plan to split its devices unit. Effissimo said in a regulatory filing on Thursday it had signed a confirmation letter da…