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>

src/cmd/compile/internal/gc/walk.go

@@ -20,7 +20,7 @@ func walk(fn *Node) {
 	Curfn = fn
 
 	if Debug['W'] != 0 {
-		s := fmt.Sprintf("\nbefore %v", Curfn.Func.Nname.Sym)
+		s := fmt.Sprintf("\nbefore walk %v", Curfn.Func.Nname.Sym)
 		dumplist(s, Curfn.Nbody)
 	}
 
@@ -473,7 +473,7 @@ func walkexpr(n *Node, init *Nodes) *Node {
 	lno := setlineno(n)
 
 	if Debug['w'] > 1 {
-		Dump("walk-before", n)
+		Dump("before walk expr", n)
 	}
 
 	if n.Typecheck() != 1 {
@@ -1763,7 +1763,7 @@ opswitch:
 	updateHasCall(n)
 
 	if Debug['w'] != 0 && n != nil {
-		Dump("walk", n)
+		Dump("after walk expr", n)
 	}
 
 	lineno = lno