Take more shortcuts in i4x4/i8x8 analysis

Take more shortcuts in i4x4/i8x8 analysis
Based on the scores of the H and V modes, rule out modes which are unlikely.
Small compression loss (0.1-0.5%) and large speed gain (10-30% faster intra analysis).
Not enabled in slower encoding modes.

Also make C versions of the merged SATD functions in order to eliminate branches based on their availability.