TJX Cos., the Framingham company that operates such offprice retail chains as T.J. Maxx, Marshalls, and HomeGoods, today reported an 8-percent increase in December same-store sales, a metric that Wall Street analysts watch closely as a measure of a retailer’s performance. That rise “significantly exceeded” company expectations and came despite unseasonably warm weather that cut demand for winter clothing. TJX also updated its earnings outlook and announced a two-for-one stock split.