Replace `!(o1 != 0)` with `o1 == 0` (for readability).

Found using https://go-critic.github.io/overview.html#boolExprSimplify-ref

Change-Id: I4fc035458f530973f9be15b38441ec7b5fb591ec
Reviewed-on: https://go-review.googlesource.com/123377
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>