- First pass encoding time reduces ~10.9% on i7-6700
  at 100 frames, 1080p.
- avx2 works for coeff number >= 8 cases; coeff number < 8
  case will be implemented by sse2.
- Unit test is added type B/FP/DC.

Change-Id: Ibe5b7807c64e6dfc2d59c470ed50a6e8ca94ef7c