fix SCCs containing mixture of universes

And add a test showing a universe violation getting caught.

src/test/ui/nll/relate_tys/universe-violation.rs

@@ -0,0 +1,17 @@
+// Test that the NLL `relate_tys` code correctly deduces that a
+// function returning either argument CANNOT be upcast to one
+// that returns always its first argument.
+//
+// compile-flags:-Zno-leak-check
+
+#![feature(nll)]
+
+fn make_it() -> fn(&'static u32) -> &'static u32 {
+    panic!()
+}
+
+fn main() {
+    let a: fn(_) -> _ = make_it();
+    let b: fn(&u32) -> &u32 = a;
+    drop(a);
+}

src/test/ui/nll/relate_tys/universe-violation.stderr

@@ -0,0 +1,14 @@
+error: higher-ranked subtype error
+  --> $DIR/universe-violation.rs:14:9
+   |
+LL |     let a: fn(_) -> _ = make_it();
+   |         ^
+
+error: higher-ranked subtype error
+  --> $DIR/universe-violation.rs:15:9
+   |
+LL |     let b: fn(&u32) -> &u32 = a;
+   |         ^
+
+error: aborting due to 2 previous errors
+