ssa: fix goTypes.cvtNamed recurs for typeargs

cl/_testgo/tprecurfn/in.go

@@ -0,0 +1,11 @@
+package main
+
+type My[T any] struct {
+	fn   func(n T)
+	next *My[T]
+}
+
+func main() {
+	m := &My[int]{next: &My[int]{fn: func(n int) { println(n) }}}
+	m.next.fn(100)
+}