cmd/gc: check for initialization cycles in method values

Fixes #7960.

LGTM=rsc
R=rsc
CC=golang-codereviews, gri
https://golang.org/cl/159800045

test/fixedbugs/issue6703x.go

@@ -0,0 +1,21 @@
+// 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.
+
+// Check for cycles in a pointer value's method call.
+
+package ptrmethcall
+
+type T int
+
+func (*T) pm() int {
+	_ = x
+	return 0
+}
+
+var (
+	p *T
+	x = p.pm() // ERROR "initialization loop|depends upon itself"
+)