The Senate voted 92-0 Monday to approve the $3.9 billion drought-assistance bill, a wide-ranging measure that would be the most expensive disaster-relief legislation for agriculture ever passed by Congress. The bill would extend help to hundreds of thousands of growers and ranchers. It is expected to receive final congressional approval from the House, perhaps as early as Tuesday, and President Reagan is expected to sign it quickly. Under the legislation, growers who lose more than 35 percent of their crop because of the drought would be entitled to an emergency payment totaling 65 percent of their expected federal subsidy or of the anticipated market price of the crop, up to $100,000.