Handle single ref mv pair in the candidate list for compound mode

This commit considers the case where a single reference motion
vector pair is found in the candidate list. It treats this pair
as the effective motion vector for nearestmv mode. This improves
the coding performance by 0.06% for stdhd sets.

Change-Id: I9ce12f456b52912933e05c18c3841a78c26155d2
2 files changed