1. Search for block8x16/block16x8 uses block8x8's search results.
2. Check block4x4 only if block8x8 is chosen. (This hurts quality,
   which will be improved in another check-in.)
3. In block4x4 search, the previous block's result is used as
   MV predictor for next block.

This change improves performance.

Change-Id: I9dc089007ca08129fb6c11fe7692777ebb8647b0