In the rd loop, check the perf of obmc, whose mv is copied from regular
inter predictor, when wedge interinter is better than regular inter
(previously it will force allow_obmc = 0). The condition of the early
termination before this step is relaxed to avoid skipping too many obmc
predictions. The rates of the overhead are properly calculated for these tools.

The logic of the bitstream syntax:
(a single ref) the interintra flag is sent first, only if it is 0, we
send the obmc flag;
(compound refs) the obmc flag is sent first, only if it is 0, we send
the wedge interinter flag

Coding gain
lowres: 0.428% (2.287%->2.715%)

Change-Id: I5f3a34640b398e313cbf84235c9fe2073eb2173f