Gcc idiocy fixes related to filter_mb_edge*.

Change order of operands as gcc uses a hardcoded register per operand it seems
even for static functions
thus reducing unneeded moved (now functions try to pass the same argument in
the same spot).
Change signed int to unsigned int for array indexes as signed requires signed
extension while unsigned is free.
move the +52 up and merge it where it will end as a lea instruction, gcc always
splits the 52 out there turning the free +52 into an expensive one otherwise.
The changed code becomes a little faster.

Originally committed as revision 21375 to svn://svn.ffmpeg.org/ffmpeg/trunk