Minor cleanup.

This is the only such instance in the compiler.

Change-Id: I4e8ecde57d71867c7e1ac4d17e2154a91dd262b0
Reviewed-on: https://go-review.googlesource.com/39209
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>