cmd/gc: avoid infinite recursion on invalid recursive type

Fixes #8507.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews, r
https://golang.org/cl/144560043

src/cmd/gc/align.c

@@ -119,8 +119,10 @@ dowidth(Type *t)
 	if(t->width == -2) {
 		lno = lineno;
 		lineno = t->lineno;
-		if(!t->broke)
+		if(!t->broke) {
+			t->broke = 1;
 			yyerror("invalid recursive type %T", t);
+		}
 		t->width = 0;
 		lineno = lno;
 		return;

src/cmd/gc/subr.c

@@ -529,7 +529,8 @@ algtype1(Type *t, Type **bad)
 	
 	if(bad)
 		*bad = T;
-
+	if(t->broke)
+		return AMEM;
 	if(t->noalg)
 		return ANOEQ;
 

test/fixedbugs/issue8507.go

+// errorcheck
+
+// Copyright 2014 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 8507
+// used to call algtype on invalid recursive type and get into infinite recursion
+
+package p
+
+type T struct{ T } // ERROR "invalid recursive type T"
+
+func f() {
+	println(T{} == T{})
+}