The compound mode context model depends on the number of 8x8
blocks coded in the newmv mode under opt-ref-mv. This commit makes
the codec account for all the newmv modes in both single and
compound settings for that purpose. It only affects changes under
opt-ref-mv.

It improves the midres coding performance by 0.08%.

BUG=aomedia:1358

Change-Id: I0899cbb31e0001d958677128bcc94b063b449817