cmd/compile: allow VARDEF at top level

This was missed as part of adding a top-level VARDEF
for stack tracing (CL 134156).

Fixes #28055

Change-Id: Id14748dfccb119197d788867d2ec6a3b3c9835cf
Reviewed-on: https://go-review.googlesource.com/c/140304
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>

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

@@ -211,6 +211,7 @@ func walkstmt(n *Node) *Node {
 		ODCLCONST,
 		ODCLTYPE,
 		OCHECKNIL,
+		OVARDEF,
 		OVARKILL,
 		OVARLIVE:
 		break

test/fixedbugs/issue28055.go

+// compile
+
+// Copyright 2018 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.
+
+// Make sure VARDEF can be a top-level statement.
+
+package p
+
+func f() {
+	var s string
+	var as []string
+	switch false && (s+"a"+as[0]+s+as[0]+s == "") {
+	}
+}