Add AVX2 intrinsic for variance function for block width 8

Added AVX2 intrinsic optimization for the following functions
1. vpx_variance8x4
2. vpx_variance8x8
3. vpx_variance8x16

This is a bit-exact change.

                 Instruction Count
cpu   Resolution   Reduction(%)
 0       LOWRES2      0.698
 0       MIDRES2      0.577
 0        HDRES2      0.469
 0       Average      0.582

Change-Id: Iae8fdf9344fd012cda4955ed140633141d60ba86