runtime: simplify the control flow in sweepone

Ending a loop with a break is confusing. Rewrite the loop so the
default behavior is to loop and then do the "post-loop" work outside
of the loop.

Change-Id: Ie49b4132541dfb5124c31a8163f2c883aa4abc75
Reviewed-on: https://go-review.googlesource.com/138155


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>