cmd/6g: cosmetic improvements to regopt debugging.
cmd/6g: cosmetic improvements to regopt debugging.

R=rsc, golang-dev
CC=golang-dev
http://codereview.appspot.com/6528044
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go