This commit enables the encoder to skip checking ALTREF inter modes
in ARF coding, if the predicted motion vectors suggest that the
GOLDEN_FRAME provides higher prediction accuracy than ALTREF_FRAME.

It improves the speed 3 encoding speed by about 5%, at the expense
of compression performance loss -0.041% and -0.225% for derf and
stdhd, respectively.

pedestrian_area 1080p 2000 kbps
66705 b/f, 40.909 dB, 118738 ms ->
66732 b/f, 40.908 dB, 113688 ms

old_town_cross 720p 1500 kbps
14427 b/f, 36.256 dB, 62746 ms ->
14412 b/f, 36.252 dB, 60690 ms

blue_sky 1080p 1500 kbps
51026 b/f, 35.897 dB, 73310 ms ->
50921 b/f, 35.893 dB, 70406 ms

bus CIF 1000 kbps
21301 b/f, 34.841 dB, 7326 ms ->
21248 b/f, 34.837 dB, 7196 ms

Change-Id: I76cf88b4d655e1ee3c0cb03c8a5745493040e8d2