The Securities and Exchange Commission, after having hit Goldman Sachs Group Inc. with a civil fraud charge, is investigating whether other mortgage deals arranged by some of Wall Street’s biggest firms may have crossed the line into misleading investors. The SEC’s case against Goldman Friday has exposed an open secret on Wall Street: As the housing market began to wobble a few years back, some big financial firms designed products aimed at allowing key clients, such as hedge funds, to bet on a sharp housing downturn.