TOKYO (Reuters) - SoftBank Group Corp has built a global conglomerate of telecoms and tech companies on an ever-growing mountain of debt that major rating agencies categorise as junk.SoftBank's decision on Oct. 24 to spend more than $10 billion to bail out its office-sharing portfolio company WeWork has raised concerns about the group's investment and governance standards. Jefferies downgraded SoftBank stock, saying the rescue is a case of "throwing good money after bad".BIGGEST ISSUERMasayoshi..