cmd/compile: early typecheck top level OAS2 nodes

Fixes #10977.

Change-Id: I706c953c16daad48595c7fae2d82124614dfc3ad
Reviewed-on: https://go-review.googlesource.com/10780Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -370,7 +370,7 @@ func Main() {
 	defercheckwidth()
 
 	for l := xtop; l != nil; l = l.Next {
-		if l.N.Op != ODCL && l.N.Op != OAS {
+		if l.N.Op != ODCL && l.N.Op != OAS && l.N.Op != OAS2 {
 			typecheck(&l.N, Etop)
 		}
 	}
@@ -378,7 +378,7 @@ func Main() {
 	// Phase 2: Variable assignments.
 	//   To check interface assignments, depends on phase 1.
 	for l := xtop; l != nil; l = l.Next {
-		if l.N.Op == ODCL || l.N.Op == OAS {
+		if l.N.Op == ODCL || l.N.Op == OAS || l.N.Op == OAS2 {
 			typecheck(&l.N, Etop)
 		}
 	}

test/fixedbugs/issue10977.go

+// compile
+
+// Copyright 2015 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 p
+
+type T struct{}
+
+var (
+	t = T{}
+	u = t.New()
+)
+
+func x(T) (int, int) { return 0, 0 }
+
+var _, _ = x(u)
+
+func (T) New() T { return T{} }