Fix 32-bit unsigned integer overflow in block_variance
Fix 32-bit unsigned integer overflow in block_variance

When compiled for High Bitdepth SSE can overflow 32-bit unsigned
integer, so change it to 64 bit. Also fixing unit/int mismatch of sum

BUG=webm:1601

Change-Id: Ib576ed1d5579b0c2b4661058aa64119560b652bf

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