Early skip of compound search
Early skip of compound search

Skip compound mode search if current mode cost is already larger than
ref_best_rd.

Tested BasketballDrill_832x480_50 (20 frames, bitrate=800, Speed 1),
it is about 1.9% faster and identical result.

Change-Id: Ic5471e594d1faed978b144ff6b3db9ef894a9094

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom