This speed feature will skip searching the directional intra prediction
modes D63, D117, D27, D153 if the best intra mode so far is not one of
the diagonal, horizontal or vertical directions closest to the respective
directions being tested. In other words, this implements a sort of
binary search in the angular domain.

Speedup: about 9-10%
Results: -0.05% only on derfraw300.

Change-Id: I413584c41f2a3e8dabfbdeb40718c8fc4b1d63a2