cmd/compile: silence superfluous assignment error message

Avoid printing a second error message when a field of an undefined
variable is accessed.

Fixes #8440.

Change-Id: I3fe0b11fa3423cec3871cb01b5951efa8ea7451a
Reviewed-on: https://go-review.googlesource.com/36751Reviewed-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/typecheck.go

@@ -3213,8 +3213,8 @@ func checkassign(stmt *Node, n *Node) {
 		return
 	}
 
-	// have already complained about n being undefined
-	if n.Op == ONONAME {
+	// have already complained about n being invalid
+	if n.Type == nil {
 		return
 	}
 

test/fixedbugs/issue14010.go

@@ -11,5 +11,5 @@ package main
 
 func main() {
 	true = false // ERROR "cannot assign to true"
-	byte = 0     // ERROR "not an expression" "cannot assign to byte"
+	byte = 0     // ERROR "not an expression"
 }

test/fixedbugs/issue8440.go

+// errorcheck
+
+// Copyright 2017 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.
+
+package main
+
+func main() {
+	n.foo = 6 // ERROR "undefined: n in n.foo"
+}