cmd/compile: large ptr literals must escape

They get rewritten to NEWs, and they must be marked as escaping
so walk doesn't try to allocate them back onto the stack.

Fixes #15733

Change-Id: I433033e737c3de51a9e83a5a273168dbc9110b74
Reviewed-on: https://go-review.googlesource.com/23223
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -640,7 +640,7 @@ func esc(e *EscState, n *Node, up *Node) {
 	// "Big" conditions that were scattered around in walk have been gathered here
 	if n.Esc != EscHeap && n.Type != nil &&
 		(n.Type.Width > MaxStackVarSize ||
-			n.Op == ONEW && n.Type.Elem().Width >= 1<<16 ||
+			(n.Op == ONEW || n.Op == OPTRLIT) && n.Type.Elem().Width >= 1<<16 ||
 			n.Op == OMAKESLICE && !isSmallMakeSlice(n)) {
 		if Debug['m'] > 2 {
 			Warnl(n.Lineno, "%v is too large for stack", n)

test/fixedbugs/issue15733.go

@@ -0,0 +1,23 @@
+// compile
+
+// Copyright 2016 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
+
+type S struct {
+	a [1 << 16]byte
+}
+
+func f1() {
+	p := &S{}
+	_ = p
+}
+
+type T [1 << 16]byte
+
+func f2() {
+	p := &T{}
+	_ = p
+}