Make sure that the lack of an lvalue doesn't
cause extra side-effects.

Updates #18661
Updates #18739

Change-Id: I52eb4b4a5c6f8ff5cddd2115455f853c18112c19
Reviewed-on: https://go-review.googlesource.com/36126
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>