cmd/compile: make -W and -w headers and footers clearer

-W and -w turn on printing of Nodes for both order and walk.
I have found their output mildly incomprehensible for years.
Improve it, at long last.

Change-Id: Ia05d77e59aa741c2dfc9fcca07f45019420b655e
Reviewed-on: https://go-review.googlesource.com/114520


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>