cmd/compile/internal/gc: convert Label.Used to bool

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>

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

@@ -729,7 +729,7 @@ func gen(n *Node) {
 				break
 			}
 
-			lab.Used = 1
+			lab.Used = true
 			if lab.Breakpc == nil {
 				Yyerror("invalid break label %v", n.Left.Sym)
 				break
@@ -754,7 +754,7 @@ func gen(n *Node) {
 				break
 			}
 
-			lab.Used = 1
+			lab.Used = true
 			if lab.Continpc == nil {
 				Yyerror("invalid continue label %v", n.Left.Sym)
 				break
@@ -994,7 +994,7 @@ func checklabels() {
 			continue
 		}
 
-		if lab.Use == nil && lab.Used == 0 {
+		if lab.Use == nil && !lab.Used {
 			yyerrorl(int(lab.Def.Lineno), "label %v defined and not used", lab.Sym)
 			continue
 		}

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

@@ -217,7 +217,6 @@ type Type struct {
 }
 
 type Label struct {
-	Used uint8
 	Sym  *Sym
 	Def  *Node
 	Use  []*Node
@@ -228,6 +227,8 @@ type Label struct {
 	Labelpc  *obj.Prog // pointer to code
 	Breakpc  *obj.Prog // pointer to code
 	Continpc *obj.Prog // pointer to code
+
+	Used bool
 }
 
 type InitEntry struct {