Condition use of minmax in variance partition on speed setting.
Condition use of minmax in variance partition on speed setting.

For non-rd variance partition: only allow minmax computation
(which currently has no arm-neon optimization) for speeds < 8.

Performance loss is small: On RTC set with speed 8, few clips lose ~2/3%,
average loss is < 1%.

Change-Id: Ia9414f4d0b77dc83c3e73ca8de5d903f64b425ce

openpeer/libvpx_new,thdav/aom,mbebenita/aom,luctrudeau/aom,luctrudeau/aom,openpeer/libvpx_new,mwgoldsmith/vpx,webmproject/libvpx,webmproject/libvpx,luctrudeau/aom,jmvalin/aom,webmproject/libvpx,openpeer/libvpx_new,ittiamvpx/libvpx-1,shacklettbp/aom,luctrudeau/aom,Topopiccione/libvpx,smarter/aom,Topopiccione/libvpx,jmvalin/aom,mbebenita/aom,mwgoldsmith/libvpx,thdav/aom,ShiftMediaProject/libvpx,thdav/aom,ittiamvpx/libvpx-1,mwgoldsmith/libvpx,thdav/aom,jmvalin/aom,ShiftMediaProject/libvpx,thdav/aom,mwgoldsmith/libvpx,openpeer/libvpx_new,smarter/aom,GrokImageCompression/aom,openpeer/libvpx_new,shacklettbp/aom,GrokImageCompression/aom,mwgoldsmith/vpx,mwgoldsmith/vpx,shacklettbp/aom,jmvalin/aom,GrokImageCompression/aom,ShiftMediaProject/libvpx,thdav/aom,mbebenita/aom,jmvalin/aom,GrokImageCompression/aom,ittiamvpx/libvpx-1,mwgoldsmith/vpx,mwgoldsmith/vpx,mwgoldsmith/vpx,smarter/aom,smarter/aom,Topopiccione/libvpx,webmproject/libvpx,smarter/aom,shacklettbp/aom,ittiamvpx/libvpx-1,mbebenita/aom,mwgoldsmith/libvpx,mbebenita/aom,mwgoldsmith/libvpx,GrokImageCompression/aom,mwgoldsmith/libvpx,mbebenita/aom,luctrudeau/aom,ittiamvpx/libvpx-1,ShiftMediaProject/libvpx,luctrudeau/aom,Topopiccione/libvpx,shacklettbp/aom,webmproject/libvpx,ShiftMediaProject/libvpx,shacklettbp/aom,openpeer/libvpx_new,mbebenita/aom,mbebenita/aom,Topopiccione/libvpx,mbebenita/aom,smarter/aom,webmproject/libvpx,ittiamvpx/libvpx-1,jmvalin/aom,Topopiccione/libvpx,GrokImageCompression/aom