Also replace repeated `or` clauses with the Python idiomatic list
operator `in`.

Change-Id: I4b178f93eb92996d8b5449ee5d252543624aed9e
Reviewed-on: https://go-review.googlesource.com/111215
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>