The state’s top economic development official, who has pushed lawmakers to legalize gaming and touted the economic benefits of resort casinos, bought stock in two Las Vegas gaming companies last year, according to records filed with the S tate Ethics Commission. Gregory Bialecki, secretary of housing and economic development, said he held more than $17,000 stock in Las Vegas Sands Corp. and Wynn Resorts Ltd., both of which have expressed interest in building casinos in Massachusetts.