another interface conversion test

R=r
DELTA=21  (21 added, 0 deleted, 0 changed)
OCL=29212
CL=29249

test/interface/convert2.go

+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2009 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.
+
+// Check that static interface conversion of
+// interface value nil succeeds.
+
+package main
+
+type R interface { R(); }
+type RW interface { R(); W(); }
+
+var e interface {}
+var r R;
+var rw RW;
+
+func main() {
+	r = r;
+	r = rw;
+	e = r;
+	e = rw;
+	rw = rw;
+}