cmd/compile: merge Node.Opt and Node.Val behind access methods

$ sizeof -p cmd/compile/internal/gc Node
Node 144
$

Change-Id: I688e3790964fe42f48c19f697ec38094a92fe1c1
Reviewed-on: https://go-review.googlesource.com/10531
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/src/cmd/compile/internal/gc/dcl.go b/src/cmd/compile/internal/gc/dcl.go
index 7cdf3cc..cba8e21 100644
--- a/src/cmd/compile/internal/gc/dcl.go
+++ b/src/cmd/compile/internal/gc/dcl.go
@@ -833,10 +833,10 @@
 		f.Broke = 1
 	}
 
-	switch n.Val.Ctype() {
+	switch n.Val().Ctype() {
 	case CTSTR:
 		f.Note = new(string)
-		*f.Note = n.Val.U.(string)
+		*f.Note = n.Val().U.(string)
 
 	default:
 		Yyerror("field annotation must be string")
@@ -942,7 +942,7 @@
 		Fatal("interfacefield: oops %v\n", n)
 	}
 
-	if n.Val.Ctype() != CTxxx {
+	if n.Val().Ctype() != CTxxx {
 		Yyerror("interface method cannot have annotation")
 	}