Skip checking compound modes with same sign bias for sub8x8
Skip checking compound modes with same sign bias for sub8x8

Drop the check of compound modes where the two reference frames
share the same reference frame sign bias in sub8x8 coding blocks.

Change-Id: I47b45256582b2b5ea1372c9130d8f28cd226a29c

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