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/gsubr.go b/src/cmd/compile/internal/gc/gsubr.go
index 6dd2bc9..14dc927 100644
--- a/src/cmd/compile/internal/gc/gsubr.go
+++ b/src/cmd/compile/internal/gc/gsubr.go
@@ -408,26 +408,26 @@
 		if Thearch.Thechar == '8' {
 			a.Width = 0
 		}
-		switch n.Val.Ctype() {
+		switch n.Val().Ctype() {
 		default:
 			Fatal("naddr: const %v", Tconv(n.Type, obj.FmtLong))
 
 		case CTFLT:
 			a.Type = obj.TYPE_FCONST
-			a.Val = mpgetflt(n.Val.U.(*Mpflt))
+			a.Val = mpgetflt(n.Val().U.(*Mpflt))
 
 		case CTINT, CTRUNE:
 			a.Sym = nil
 			a.Type = obj.TYPE_CONST
-			a.Offset = Mpgetfix(n.Val.U.(*Mpint))
+			a.Offset = Mpgetfix(n.Val().U.(*Mpint))
 
 		case CTSTR:
-			datagostring(n.Val.U.(string), a)
+			datagostring(n.Val().U.(string), a)
 
 		case CTBOOL:
 			a.Sym = nil
 			a.Type = obj.TYPE_CONST
-			a.Offset = int64(obj.Bool2int(n.Val.U.(bool)))
+			a.Offset = int64(obj.Bool2int(n.Val().U.(bool)))
 
 		case CTNIL:
 			a.Sym = nil