Forget Internet IPOs: One of the best investments of the past couple of years was a bankrupt airline. When American Airlines parent AMR Corp. filed for bankruptcy protection in November 2011, its stock plunged to 20 cents a share and was soon delisted from the New York Stock Exchange. Today, as American prepares to close a merger with US Airways Group Inc., the stock trades at just below $11, and a small group of investors who bet on it when it was flying low are poised to reap one of the biggest bankruptcy windfalls in years. That is thanks in part to a little-noticed quirk in the deal that means their holdings could translate into much larger stakes than previously expected in the combined airline, to be called American Airlines Group Inc.