ssa: fix closure type

cl/_testrt/closureiface/in.go

@@ -0,0 +1,14 @@
+package main
+
+func main() {
+	var m int = 200
+	fn := func(n int) int {
+		return m + n
+	}
+	var i any = fn
+	f, ok := i.(func(int) int)
+	if !ok {
+		panic("error")
+	}
+	println(f(100))
+}