Add AVX2 version of vpx_convolve8_avg.
vpx_convolve8_avg works by first running a normal horizontal filter then a vertical filter averages at the end. The added vpx_convolve8_avg_avx2 calls pre-existing AVX2 code for the horizontal step. vpx_convolve8_avg_vert_avx2 is also added, but only uses ssse3 code. Change-Id: If5160c0c8e778e10de61ee9bf42ee4be5975c983
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