Unroll main loop so the edge==0 case is seperate.

This allows many things to be simplified away.
h264 decoder is overall 1% faster with a mbaff sample and
0.1% slower with the cathedral sample, probably because the slow loop
filter code must be loaded into the code cache for each first MB of each
row but isnt used for the following MBs.

Originally committed as revision 21493 to svn://svn.ffmpeg.org/ffmpeg/trunk