VP9: Add greedy version of av1_optimize_b().

VP9: Add greedy version of av1_optimize_b().

This was ported from the greedy version in AV1, written by Dake He
(dkhe@google.com).
See:
https://aomedia.googlesource.com/aom/+/master/av1/encoder/encodemb.c#137

Greedy version is disabled by default, but can be picked by setting
USE_GREEDY_OPTIMIZE_B to 1.
To be enabled by default later.

This is both faster and better in terms of compression.

Compression Improvement:
------------------------
lowres: -0.119
midres: -0.064
hdres:  -0.405

Speed Improvement:
------------------
(Based on encode time of 3 videos of different difficulties at
3 different target bitrates)
With --cpu-used=0: 0.38% to 5.55% faster
With --cpu-used=1: 0.24% to 2.79% faster
With --cpu-used=2: 0.29% to 1.46% faster

Change-Id: Ia7a23b3b244ad8eb253ac9e43cd03c5e021d2635