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>
diff --git a/src/cmd/compile/internal/gc/go.go b/src/cmd/compile/internal/gc/go.go
index 8c7328f..91838ff 100644
--- a/src/cmd/compile/internal/gc/go.go
+++ b/src/cmd/compile/internal/gc/go.go
@@ -217,7 +217,6 @@
 }
 
 type Label struct {
-	Used uint8
 	Sym  *Sym
 	Def  *Node
 	Use  []*Node
@@ -228,6 +227,8 @@
 	Labelpc  *obj.Prog // pointer to code
 	Breakpc  *obj.Prog // pointer to code
 	Continpc *obj.Prog // pointer to code
+
+	Used bool
 }
 
 type InitEntry struct {