cmd/compile: fix isStaticCompositeLiteral reports wrong for struct field

golang.org/cl/174498 add ONAME case to isStaticCompositeLiteral, to
detect global variable as compile-time constant.

It does report wrong for struct field, e.g:

	o := one{i: two{i: 42}.i}

field i in two{i: 42} was reported as static composite literal, while it
should not.

In general, adding ONAME case for isStaticCompositeLiteral is probably
wrong.

Fixes #31782

Change-Id: Icde7d43bbb002b75df5c52b948b7126a4265e07b
Reviewed-on: https://go-review.googlesource.com/c/go/+/174837Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -650,8 +650,6 @@ func getdyn(n *Node, top bool) initGenType {
 // isStaticCompositeLiteral reports whether n is a compile-time constant.
 func isStaticCompositeLiteral(n *Node) bool {
 	switch n.Op {
-	case ONAME:
-		return n.Class() == PEXTERN && n.Name != nil && n.Name.Readonly()
 	case OSLICELIT:
 		return false
 	case OARRAYLIT:

test/fixedbugs/issue31782.go

+// run
+
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Check static composite literal reports wrong for struct
+// field.
+
+package main
+
+type one struct {
+	i interface{}
+}
+
+type two struct {
+	i interface{}
+	s []string
+}
+
+func main() {
+	o := one{i: two{i: 42}.i}
+	println(o.i.(int))
+}

test/fixedbugs/issue31782.out

+42