sse2 & ssse3 versions of hadamard. unroll and inline diff_pixels.

core2: before mmx2=193 cycles. after mmx2=174 sse2=122 ssse3=115 (cycles).
k8: before mmx2=205. after mmx2=184 sse2=180.
p4: before mmx2=342. after mmx2=314 sse2=309.

Originally committed as revision 9000 to svn://svn.ffmpeg.org/ffmpeg/trunk