optimize sign decoding code in decode_residual()

optimize sign decoding code in decode_residual()
x86 is 4% faster on P3
C sign stuff + x86 code for everything else is also faster then before (sorry forgot to test pure C)
... and if i replace the second occurance of the sign decoding in decode_residual by the asm too then everything gets slower iam starting to think that it might be best to write the whole function in asm, playing this avoid random deoptimizations game with gcc is not fun at all

Originally committed as revision 6732 to svn://svn.ffmpeg.org/ffmpeg/trunk