add bug246 (issue 492)

R=r
CC=golang-dev
https://golang.org/cl/186216

test/bugs/bug246.go

+// $G $D/$F.go || echo BUG: bug246
+
+// 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.
+
+package main
+
+import "unsafe"
+
+func main() {
+	// works
+	addr := uintptr(0)
+	_ = (*int)(unsafe.Pointer(addr))
+
+	// fails
+	_ = (*int)(unsafe.Pointer(uintptr(0)))
+}

test/golden.out

@@ -149,3 +149,8 @@ panic PC=xxx
 bad map check 13 false false
 panic PC=xxx
 BUG: tuple evaluation order
+
+=========== bugs/bug246.go
+bugs/bug246.go:17: cannot convert 0 to type unsafe.Pointer
+bugs/bug246.go:17: cannot convert 0 (type uintptr) to type *int in conversion
+BUG: bug246