cmd/gc: check for initialization cycles in method values

Fixes #7960.

LGTM=rsc
R=rsc
CC=golang-codereviews, gri
https://golang.org/cl/159800045
diff --git a/test/fixedbugs/issue6703s.go b/test/fixedbugs/issue6703s.go
new file mode 100644
index 0000000..6aa2848
--- /dev/null
+++ b/test/fixedbugs/issue6703s.go
@@ -0,0 +1,18 @@
+// 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 method expression.
+
+package ptrmethexpr
+
+type T int
+
+func (*T) pm() int {
+	_ = x
+	return 0
+}
+
+var x = (*T).pm // ERROR "initialization loop|depends upon itself"