[release-branch.go1.4] cmd/gc: don't recurse infinitely when a recursive type...

[release-branch.go1.4] cmd/gc: don't recurse infinitely when a recursive type references itself more than once

Fixes #9432

Change-Id: I08c92481afa7c7fac890aa780efc1cb2fabad528
Reviewed-on: https://go-review.googlesource.com/2115


Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>
(cherry picked from commit fcff3ba7)
Reviewed-on: https://go-review.googlesource.com/5004


Run-TryBot: Andrew Gerrand <adg@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>