gc: fix conversion of user-defined string type to []byte

Fixes #1709.

R=ken2
CC=golang-dev
https://golang.org/cl/4438068

src/cmd/gc/walk.c

@@ -1356,7 +1356,7 @@ walkexpr(Node **np, NodeList **init)
 
 	case OSTRARRAYBYTE:
 		// stringtoslicebyte(string) []byte;
-		n = mkcall("stringtoslicebyte", n->type, init, n->left);
+		n = mkcall("stringtoslicebyte", n->type, init, conv(n->left, types[TSTRING]));
 		goto ret;
 
 	case OSTRARRAYRUNE:

test/fixedbugs/bug333.go

+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2011 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.
+
+// Issue 1709
+
+package main
+
+func main() {
+       type Ts string
+       var ts Ts
+       _ = []byte(ts)
+}
+
+/*
+bug333.go:14: cannot use ts (type Ts) as type string in function argument
+*/