Put iterative motion search under speed control

Enable iterative motion search for compound inter-inter prediction
of block sizes 4x4/4x8/8x4 only when best coding quality is selected.
The iterative motion search provides about 0.1% gains for derf and
stdhd at this point, at the expense of longer runtime.

Change-Id: Idc03e7f827e51f1bb8d269bc3752ee297a6bbfe5