The Tony-winning musical “Jerome Robbins’ Broadway” has taken another detour en route to the Shubert Theatre here. The hit Broadway show, which had been scheduled to open next April, has been postponed until May, 1991, the producers announced Thursday. The troupe that will bring the show here will first play Japan in February, 1991. One of the show’s original producers is a Japanese firm, the Suntory Corp. A brief statement from the producers offered no explanation for the postponement but assured audiences “with certainty” that the scheduled openings will take place. Ahmanson Theatre subscribers, who have been offered the Robbins show as part of their season, should stay tuned.