By Gertrude Chavez-Dreyfuss NEW YORK, May 21 (Reuters) – Surging U.S. Treasury yields have prompted mortgage investors to hedge the loans in their portfolios by selling government debt, a shift that probably exacerbated the bond selloff this week and added to the biggest rate spike in a year. U.S. yields have climbed since higher-than-expected inflation reports for April prompted investors to expect the Federal Reserve will hike interest rates t…