SSE2/SSSE3 optimizations for build_predictors_mbuv{,_s}().

decoding

before
10.425
10.432
10.423
=10.426

after:
10.405
10.416
10.398
=10.406, 0.2% faster

encoding

before
14.252
14.331
14.250
14.223
14.241
14.220
14.221
=14.248

after
14.095
14.090
14.085
14.095
14.064
14.081
14.089
=14.086, 1.1% faster

Change-Id: I483d3d8f0deda8ad434cea76e16028380722aee2