Use a neural-network-based binary classifier to predict the first split
decision on the highest level of the TX size RD search tree. Depending
on how confident we are in the prediction we either keep full unmodified
TX size search or use the largest possible TX size and stop any further
search.

Average speed-up: 3-4%
Quality loss (lowres): 0.062%
Quality loss (midres): 0.018%

Change-Id: I64c0317db74cbeddfbdf772147c43e99e275891f