subpel variance: add speed test
subpel variance: add speed test

Was used to verify assembly speed versus an attempt to rewrite
in intrinsics.

Change-Id: I011fe5494334b8fcda04b9d54c6093dbcfc55710

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