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

vp9/encoder/vp9_rdopt.c

@@ -1379,7 +1379,7 @@ static void rd_check_segment_txsize(VP9_COMP *cpi, MACROBLOCK *x,
 
           // adjust src pointers
           mi_buf_shift(x, i);
-          if (cpi->sf.comp_inter_joint_search) {
+          if (cpi->compressor_speed == 0 && cpi->sf.comp_inter_joint_search) {
             iterative_motion_search(cpi, x, bsize, frame_mv[this_mode],
                                     scaled_ref_frame,
                                     mi_row, mi_col, seg_mvs[i]);