President Reagan signed landmark bipartisan legislation Tuesday aimed at banning housing discrimination, calling it the "most important civil rights legislation in 20 years." In a Rose Garden ceremony, the president noted the bill not only eliminates discrimination on the basis of race, but deals with denial of housing to the handicapped, families with children and the elderly. After years of complaints about continuing discrimination despite the 1968 Fair Housing Act, the reform bill was adopted with broad bipartisan support from Congress and the real estate industry. The Senate approved the legislation on a 94-3 vote Aug. 8, the same day the House endorsed it 376-23.