cmd/compile: post CL 20089 cleanups

cmd/compile: post CL 20089 cleanups

This CL addresses some issues noted during CL 20089.

Change-Id: I4e91a8077c07a571ccc9c004278672eb951c5104
Reviewed-on: https://go-review.googlesource.com/20181
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dave Cheney <dave@cheney.net>
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>