Modify motion estimation for SPLITMV mode
Modify motion estimation for SPLITMV mode

1. Search for block8x16/block16x8 uses block8x8's search results.
2. Check block4x4 only if block8x8 is chosen. (This hurts quality,
   which will be improved in another check-in.)
3. In block4x4 search, the previous block's result is used as
   MV predictor for next block.

This change improves performance.

Change-Id: I9dc089007ca08129fb6c11fe7692777ebb8647b0

shareefalis/libvpx,mwgoldsmith/libvpx,felipebetancur/libvpx,mwgoldsmith/vpx,WebRTC-Labs/libvpx,Acidburn0zzz/webm.libvpx,running770/libvpx,vasilvv/esvp8,abwiz0086/webm.libvpx,stewnorriss/libvpx,WebRTC-Labs/libvpx,stewnorriss/libvpx,Topopiccione/libvpx,ittiamvpx/libvpx,Distrotech/libvpx,reimaginemedia/webm.libvpx,kleopatra999/webm.libvpx,shareefalis/libvpx,Maria1099/webm.libvpx,GrokImageCompression/aom,jmvalin/aom,abwiz0086/webm.libvpx,Topopiccione/libvpx,jdm/libvpx,pcwalton/libvpx,rikaunite/gst-opera_libvpx,Acidburn0zzz/webm.libvpx,Laknot/libvpx,goodleixiao/vpx,jmvalin/aom,reimaginemedia/webm.libvpx,Topopiccione/libvpx,stewnorriss/libvpx,vasilvv/esvp8,abwiz0086/webm.libvpx,n4t/libvpx,kim42083/webm.libvpx,iniwf/webm.libvpx,jacklicn/webm.libvpx,running770/libvpx,GrokImageCompression/aom,mbebenita/aom,turbulenz/libvpx,webmproject/libvpx,genesi/libvpx0,Suvarna1488/webm.libvpx,webmproject/libvpx,cinema6/libvpx,enmasse-entertainment/libvpx,VTCSecureLLC/libvpx,Suvarna1488/webm.libvpx,zofuthan/libvpx,zofuthan/libvpx,kalli123/webm.libvpx,jacklicn/webm.libvpx,n4t/libvpx,vasilvv/esvp8,turbulenz/libvpx,WebRTC-Labs/libvpx,smarter/aom,pcwalton/libvpx,thdav/aom,Laknot/libvpx,rikaunite/gst-opera_libvpx,shareefalis/libvpx,goodleixiao/vpx,lyx2014/libvpx_c,ShiftMediaProject/libvpx,n4t/libvpx,lyx2014/libvpx_c,mwgoldsmith/libvpx,ittiamvpx/libvpx,shyamalschandra/libvpx,charup/https---github.com-webmproject-libvpx-,abwiz0086/webm.libvpx,ittiamvpx/libvpx-1,turbulenz/libvpx,turbulenz/libvpx,sanyaade-teachings/libvpx,matanbs/webm.libvpx,running770/libvpx,mbebenita/aom,luctrudeau/aom,kleopatra999/webm.libvpx,gshORTON/webm.libvpx,matanbs/vp982,luctrudeau/aom,altogother/webm.libvpx,hsueceumd/test_hui,awatry/libvpx.opencl,matanbs/vp982,cinema6/libvpx,enmasse-entertainment/libvpx,goodleixiao/vpx,Topopiccione/libvpx,rikaunite/gst-opera_libvpx,smarter/aom,Maria1099/webm.libvpx,mwgoldsmith/libvpx,kleopatra999/webm.libvpx,WebRTC-Labs/libvpx,running770/libvpx,kalli123/webm.libvpx,ittiamvpx/libvpx,Suvarna1488/webm.libvpx,shacklettbp/aom,pcwalton/libvpx,kalli123/webm.libvpx,enmasse-entertainment/libvpx,kim42083/webm.libvpx,enmasse-entertainment/libvpx,turbulenz/libvpx,cinema6/libvpx,Topopiccione/libvpx,vasilvv/esvp8,iniwf/webm.libvpx,kim42083/webm.libvpx,gshORTON/webm.libvpx,smarter/aom,shyamalschandra/libvpx,luctrudeau/aom,luctrudeau/aom,mwgoldsmith/libvpx,ShiftMediaProject/libvpx,mbebenita/aom,Distrotech/libvpx,ittiamvpx/libvpx,Suvarna1488/webm.libvpx,thdav/aom,jdm/libvpx,jdm/libvpx,ittiamvpx/libvpx,ittiamvpx/libvpx-1,webmproject/libvpx,jacklicn/webm.libvpx,reimaginemedia/webm.libvpx,Maria1099/webm.libvpx,jdm/libvpx,cinema6/libvpx,iniwf/webm.libvpx,turbulenz/libvpx,smarter/aom,hsueceumd/test_hui,stewnorriss/libvpx,matanbs/webm.libvpx,ittiamvpx/libvpx-1,Suvarna1488/webm.libvpx,matanbs/webm.libvpx,shyamalschandra/libvpx,mwgoldsmith/vpx,GrokImageCompression/aom,gshORTON/webm.libvpx,altogother/webm.libvpx,shyamalschandra/libvpx,rikaunite/gst-opera_libvpx,VTCSecureLLC/libvpx,VTCSecureLLC/libvpx,openpeer/libvpx_new,Laknot/libvpx,Laknot/libvpx,cinema6/libvpx,charup/https---github.com-webmproject-libvpx-,shareefalis/libvpx,n4t/libvpx,matanbs/vp982,GrokImageCompression/aom,VTCSecureLLC/libvpx,shacklettbp/aom,smarter/aom,Laknot/libvpx,felipebetancur/libvpx,ShiftMediaProject/libvpx,altogother/webm.libvpx,kim42083/webm.libvpx,jacklicn/webm.libvpx,iniwf/webm.libvpx,goodleixiao/vpx,vasilvv/esvp8,gshORTON/webm.libvpx,zofuthan/libvpx,matanbs/vp982,goodleixiao/vpx,reimaginemedia/webm.libvpx,Maria1099/webm.libvpx,matanbs/webm.libvpx,goodleixiao/vpx,iniwf/webm.libvpx,openpeer/libvpx_new,mwgoldsmith/vpx,thdav/aom,openpeer/libvpx_new,WebRTC-Labs/libvpx,Suvarna1488/webm.libvpx,hsueceumd/test_hui,vasilvv/esvp8,stewnorriss/libvpx,Acidburn0zzz/webm.libvpx,genesi/libvpx0,kalli123/webm.libvpx,n4t/libvpx,charup/https---github.com-webmproject-libvpx-,running770/libvpx,charup/https---github.com-webmproject-libvpx-,altogother/webm.libvpx,kim42083/webm.libvpx,liqianggao/libvpx,Maria1099/webm.libvpx,felipebetancur/libvpx,thdav/aom,felipebetancur/libvpx,thdav/aom,mwgoldsmith/vpx,genesi/libvpx0,luctrudeau/aom,pcwalton/libvpx,jmvalin/aom,kleopatra999/webm.libvpx,gshORTON/webm.libvpx,Acidburn0zzz/webm.libvpx,running770/libvpx,matanbs/webm.libvpx,Laknot/libvpx,kleopatra999/webm.libvpx,Distrotech/libvpx,turbulenz/libvpx,webmproject/libvpx,liqianggao/libvpx,cinema6/libvpx,turbulenz/libvpx,hsueceumd/test_hui,smarter/aom,jdm/libvpx,sanyaade-teachings/libvpx,shareefalis/libvpx,iniwf/webm.libvpx,pcwalton/libvpx,abwiz0086/webm.libvpx,Distrotech/libvpx,matanbs/vp982,genesi/libvpx0,mwgoldsmith/libvpx,kalli123/webm.libvpx,Topopiccione/libvpx,genesi/libvpx0,openpeer/libvpx_new,jacklicn/webm.libvpx,VTCSecureLLC/libvpx,awatry/libvpx.opencl,felipebetancur/libvpx,zofuthan/libvpx,kleopatra999/webm.libvpx,reimaginemedia/webm.libvpx,openpeer/libvpx_new,Maria1099/webm.libvpx,mbebenita/aom,shacklettbp/aom,awatry/libvpx.opencl,openpeer/libvpx_new,shacklettbp/aom,charup/https---github.com-webmproject-l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