_slot_operator_update_probe: memoize use_reduce (529660)

Memoize the results of use_reduce calls inside
_slot_operator_update_probe, in order to improve performance. With
memoization, 'emerge -puvDN @world' on one of my computers takes
22.4% less time, and results in 13.5% fewer use_reduce calls.

X-Gentoo-Bug: 529660
X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=529660