This commit re-designs the reference motion vector fetching from
spatial neighbors with different reference frames from a current
coding block. Instead of re-running through the VP9 like reference
motion vector search, it goes through the nearest top row and left
column only. Such process kicks in, if and only if the regular
reference frame match based mv search didn't find 2 or more mvs.
The search through neighboring blocks with different reference
frame will stop once 2 mvs are found.

To decide the reference mvs, it compares the reference frame types
from the two blocks. If they are on the same side, directly re-use
it. Otherwise, reverse the sign of the motion vector.

The compression performance change is in the noise range as
0.03% down.

BUG=aomedia:1372

Change-Id: Ib698d7c463f2f42c767f6ca008c8a7c84289df60