Enable SSE2 4x4 inverse ADST/DCT transform. The runtime goes from
292 cycles down to 89 cycles. Running bus_cif at 2000 kbps, the
overall runtime of speed 0 goes from 301s to 295s (2% speed-up).

Change-Id: I24098136e7fee7ab2fbf1c11755bdf2ca37f3628