Port optimize_b speed-up from vp10
Port optimize_b speed-up from vp10

This commit back ports the speed-up from vp10. It improves the
unit speed by 15%.

Change-Id: Ibe8c0e0974b03266d6abd16a41e89c3b91d8db2a

mwgoldsmith/libvpx,smarter/aom,mbebenita/aom,GrokImageCompression/aom,webmproject/libvpx,mbebenita/aom,Topopiccione/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/vpx,smarter/aom,webmproject/libvpx,mwgoldsmith/libvpx,Topopiccione/libvpx,mwgoldsmith/vpx,mbebenita/aom,GrokImageCompression/aom,ShiftMediaProject/libvpx,smarter/aom,Topopiccione/libvpx,mwgoldsmith/vpx,GrokImageCompression/aom,Topopiccione/libvpx,mwgoldsmith/libvpx,mwgoldsmith/libvpx,smarter/aom,Topopiccione/libvpx,Topopiccione/libvpx,mbebenita/aom,smarter/aom,mbebenita/aom,ShiftMediaProject/libvpx,mwgoldsmith/vpx,luctrudeau/aom,mbebenita/aom,mbebenita/aom,mwgoldsmith/vpx,luctrudeau/aom,mbebenita/aom,mbebenita/aom,smarter/aom,webmproject/libvpx,ShiftMediaProject/libvpx,luctrudeau/aom,webmproject/libvpx,luctrudeau/aom,GrokImageCompression/aom,mwgoldsmith/libvpx,mwgoldsmith/libvpx,GrokImageCompression/aom,webmproject/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/vpx,GrokImageCompression/aom,luctrudeau/aom,luctrudeau/aom,webmproject/libvpx