neon variance: process two rows of 8 at a time
neon variance: process two rows of 8 at a time

When the width is equal to 8, process two rows at a time. This doubles
the speed of 8x4 and improves 8x8 by about 20%.

8x16 was using this technique already, but still improved a little bit
with the rewrite.

Also use this for vpx_get8x8var_neon

BUG=webm:1422

Change-Id: Id602909afcec683665536d11298b7387ac0a1207

webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx