ssa: fix closure type

cl/_testgo/closure2/in.go

@@ -0,0 +1,11 @@
+package main
+
+func main() {
+	x := 1
+	f := func(i int) func(int) {
+		return func(i int) {
+			println("closure", i, x)
+		}
+	}
+	f(1)(2)
+}