[CaptureTracking] Early abort on too many uses (NFCI)

If there are too many uses, we should directly return -- there's
no point in inspecting the remaining uses in the worklist, as we
have to conservatively assume a capture anyway. This also means
that tooManyUses() gets called exactly once, rather than
potentially many times.

This restores the behavior prior to e9832dfd,
where this was accidentally changed while moving the AddUses logic
into a closure, thus making the return a return from the closure
rather than the whole function.