fix broken fastpow2
fix broken fastpow2

dhgarrette/cnn,Peilun-Li/dynet,Abasyoni/dynet,Abasyoni/dynet,kamigaito/cnn,mitchellstern/dynet,yoavg/cnn,mitchellstern/dynet,mrzu/cnn,Abasyoni/dynet,cherryc/dynet,kamigaito/cnn,dhgarrette/cnn,dhgarrette/cnn,cherryc/dynet,kaishengyao/cnn,Peilun-Li/dynet,cherryc/dynet,yoavg/cnn,yanweifu/cnn,kaishengyao/cnn,Peilun-Li/dynet