Fold the printing of the offending instruction
into the neighboring Diag call, if it is not
already present.

Change-Id: I310f1479e16a4d2a24ff3c2f7e2c60e5e2015c1b
Reviewed-on: https://go-review.googlesource.com/38714
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>