CMOV-ify vp56 arithcoder

This incantation causes gcc 4.3 to generate cmov on x86, a vastly better option
than a completely unpredictable branch.
Hopefully this carries over to newer versions and other CPUs with conditionals.
~5 cycles saved per call on a Core i7.

Originally committed as revision 23921 to svn://svn.ffmpeg.org/ffmpeg/trunk