Optimize compute_stats_highbd()
Optimize compute_stats_highbd()

Change-Id: Id10367bd80f74c2646f603741dc4ce1655d44d9e

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