choose_partitioning: Avoid computing segment_id twice.
choose_partitioning: Avoid computing segment_id twice.

1% faster remoting_perftest at speed -8.

Change-Id: I78d591e14b8f890c1e69cacb6d490dfeb9c37900

mwgoldsmith/libvpx,mwgoldsmith/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/vpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,smarter/aom,mwgoldsmith/vpx,smarter/aom,mwgoldsmith/libvpx,mwgoldsmith/vpx,mwgoldsmith/vpx,mbebenita/aom,webmproject/libvpx,luctrudeau/aom,mwgoldsmith/libvpx,Topopiccione/libvpx,GrokImageCompression/aom,webmproject/libvpx,smarter/aom,mwgoldsmith/vpx,mwgoldsmith/vpx,Topopiccione/libvpx,ittiamvpx/libvpx-1,luctrudeau/aom,mbebenita/aom,mbebenita/aom,mbebenita/aom,smarter/aom,Topopiccione/libvpx,mbebenita/aom,luctrudeau/aom,ittiamvpx/libvpx-1,Topopiccione/libvpx,GrokImageCompression/aom,ShiftMediaProject/libvpx,ittiamvpx/libvpx-1,luctrudeau/aom,luctrudeau/aom,luctrudeau/aom,smarter/aom,GrokImageCompression/aom,mbebenita/aom,ShiftMediaProject/libvpx,Topopiccione/libvpx,mbebenita/aom,mwgoldsmith/libvpx,Topopiccione/libvpx,smarter/aom,ShiftMediaProject/libvpx,webmproject/libvpx,ittiamvpx/libvpx-1,GrokImageCompression/aom,GrokImageCompression/aom,mbebenita/aom,mwgoldsmith/libvpx,ittiamvpx/libvpx-1,GrokImageCompression/aom,mbebenita/aom,ittiamvpx/libvpx-1