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/issue6703i.go

@@ -0,0 +1,20 @@
+// 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 an embedded struct literal's method value.
+
+package embedlitmethvalue
+
+type T int
+
+func (T) m() int {
+	_ = x
+	return 0
+}
+
+type E struct{ T }
+
+var x = E{}.m // ERROR "initialization loop|depends upon itself"