The stock market is hot and--no surprise--so are brokerage shares. Their 1995 performance ranks No.5 among the 20 Standard & Poor's index groups, with semiconductors on top. Discounters such as Charles Schwab do well in bull markets, thanks to big retail volume. Alex. Brown & Sons, a specialist in initial public offerings, is riding the wave of high-tech issues. Salomon Brothers, focused on trading and investment banking, is suffering from defections of stars.