Convert Label.Used to a boolean. Also move the field to the
bottom of the struct to avoid padding.

This change passes go build -toolexec 'toolstash -cmp' -a std.

Change-Id: If09ee92f9d54dce807e7b862cf771005daed810d
Reviewed-on: https://go-review.googlesource.com/14308
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>