Obtain the most likely partition range from a first pass square
block base partition search. Use the constrained partition search
region for full rate-distortion optimization search in the second
pass.

Tested on pedestrian 1080p at 2000 kbps, it makes the encoding
speed 40% faster for speed 0 and 30% faster for speed 1. The
average coding performance loss is around 0.15%.

Change-Id: Ifc83d48e6413d1b887e68cd1962084e018a2258f