- Function level improvement:
functions      sse2  avx2  percentage
idct16x16_256  365   226   38%
idct16x16_38   n/a   136   n/a
idct16x16_10   171   110   35%
idct16x16_1     34    26   23%

- Integrated in AV1 for default scan order.

Change-Id: Ieb1a8e730bea9c371ebc0e5f4a748640d8f5e921