Enable sse2 implmentation of 8x8 ADST/DCT

This commit makes use of the butterfly structure to enable the sse2
version implementation of 8x8 ADST/DCT hybrid transform coding.

The runtime of hybrid transform module goes down from 1170 cycles
to 245 cycles. Overall speed-up around 1.5%.

Change-Id: Ic808ffd21ece8a9d0410d8c0243d7b6c28ac3b3f