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