A couple more optimizations

Tried to figure out why the buildbot is just dying after ~150 tests on x64.  Works fine locally, works fine on x86 on the buildbot.  Not sure how much it's worth looking into it more, I have no idea what's going on on the buildbot.  It runs for like 7 seconds and then hangs for 6+ minutes.

Anyway, this is a slight optimization (mutating an OpArg in a loop) that cuts down a lot on the Start() methods of the regcaches.  This shaves about 0.2 seconds off the total test time, or more.

-[Unknown]