This commit reworks the SSSE3 implementation of the forward 8x8
2D-DCT. It uses a cyclic rotation approach to the temporary xmm
registers. It reduces the average cycles from 158 to 154. The SSE2
version uses 169 cycles.

Change-Id: I1b79b9642aae0ed3fb3cefb5b70246e6de5d5caa