Modify HORIZx16 macro in subpixel filter functions
Modify HORIZx16 macro in subpixel filter functions

Interleaved the instructions, reduced register dependency, and
prefetched the source data. This improved the decoder speed
by 0.6% - 2%.

Change-Id: I568067aa0c629b2e58219326899c82aedf7eccca

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aginemedia/webm.libvpx,shyamalschandra/libvpx,Topopiccione/libvpx,altogother/webm.libvpx,shacklettbp/aom,Suvarna1488/webm.libvpx,stewnorriss/libvpx,jdm/libvpx,iniwf/webm.libvpx,felipebetancur/libvpx,thdav/aom,mwgoldsmith/vpx,mbebenita/aom,kim42083/webm.libvpx,running770/libvpx,Topopiccione/libvpx,jacklicn/webm.libvpx,stewnorriss/libvpx,goodleixiao/vpx,matanbs/vp982,Suvarna1488/webm.libvpx,kalli123/webm.libvpx,pcwalton/libvpx,running770/libvpx,thdav/aom,turbulenz/libvpx,lyx2014/libvpx_c,lyx2014/libvpx_c,ittiamvpx/libvpx,felipebetancur/libvpx,smarter/aom,hsueceumd/test_hui,n4t/libvpx,gshORTON/webm.libvpx,Acidburn0zzz/webm.libvpx,shacklettbp/aom,GrokImageCompression/aom,goodleixiao/vpx,Maria1099/webm.libvpx