Comparing to lv_map experiment using the old av1_optimize_b
function, This change gives 0.8% gain on lowres and midres
datasets

Comparing to the baseline with greedy optimize_b, the lv_map
experiment will have 1.2% on lowres and 1.49% on midres.
The gains are similar to the one that under the condition of
turning off av1_optimize_b for baseline and lv_map.

Change-Id: Ie17c4ecbd9ed8418ef657a526d8363e0746b5af5