gc: fix handling of types inside function bodies

Fixes #849.
Fixes #920.

R=ken2
CC=golang-dev
https://golang.org/cl/1841042

test/fixedbugs/bug290.go

@@ -0,0 +1,15 @@
+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2010 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.
+
+// http://code.google.com/p/go/issues/detail?id=920
+
+package main
+
+type X struct { x []X }
+
+func main() {
+	type Y struct { x []Y }	// used to get invalid recursive type
+}