Skip mode search based on reference frame consistency

This commit enables the encoder to skip NEARMV and ZEROMV if the
above and left blocks have identical reference frame, and the
current reference is different from that. It reduces the runtime
of speed 3 for test sequences:
bus cif at 1000 kbps 10064 ms -> 9823 ms
pedestrian 1080p at 2000 kbps 193078 ms -> 189559 ms

The compression performance is changed by
derf  -0.085%
stdhd -0.103%

Change-Id: If304f26d42e6412152a84c3dd7b02635c38444f4