vp9: Encoding cycle reduction for speed 8.

vp9: Encoding cycle reduction for speed 8.

1. Skip golden non-zeromv and newmv-last for bsize >= 16x16 if the
temporal variance obtained from choose_partitioning is very low.
2. Skip horz and vert INTRA mode for speed 8.

This change works best on the clips with little noise and with some
motion (e.g. gips_motion which has > 5% speed up). PSNR drop is 1.78%
on rtc test set, no obvious visual quality regression found.

Change-Id: Ib43b5b20e67809d03c5a6890818ddff59e1fc94a