Unroll loop in h264_idct_add16intra_sse2(). Basically identical to r25171, this
inlines scan8[] and removes loop setup. 15% faster, 0.4% overall.

See "[PATCH] unroll loop in h264_idct_add8_sse2()" thread on ML.

Originally committed as revision 25172 to svn://svn.ffmpeg.org/ffmpeg/trunk
