cmd/gc: when expanding append inline, preserve arguments

Fixes #3369.

R=golang-dev, gri, lvd, r
CC=golang-dev
https://golang.org/cl/5876044

src/cmd/gc/walk.c

@@ -2358,6 +2358,12 @@ append(Node *n, NodeList **init)
 
 	walkexprlistsafe(n->list, init);
 
+	// walkexprlistsafe will leave OINDEX (s[n]) along if both s
+	// and n are name or literal, but those may index the slice we're
+	// modifying here.  Fix explicitly.
+	for(l = n->list; l; l=l->next)
+		l->n = cheapexpr(l->n, init);
+
 	nsrc = n->list->n;
 	argc = count(n->list) - 1;
 	if (argc < 1) {

test/fixedbugs/bug428.go

+// run
+
+// Copyright 2012 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.
+
+// Test that when the compiler expands append inline it does not
+// overwrite a value before it needs it (issue 3369).
+
+package main
+
+func main() {
+	s := make([]byte, 5, 6)
+	copy(s, "12346")
+	s = append(s[:len(s)-1], '5', s[len(s)-1])
+	if string(s) != "123456" {
+		panic(s)
+	}
+}