Noticed while looking at #20356.

Cuts 160k (1%) off of the cmd/compile binary.

Change-Id: If2397bc6971d6be9be6975048adecb0b5efa6d66
Reviewed-on: https://go-review.googlesource.com/43501
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>