add a test that types w/ def_ids compare just fine to types w/o def_ids

this is not clearly going to work due to monomorphization, which convverts
each enum<T> to a distinct "type" from POV of the shape code

src/test/run-pass/compare-generic-enums.rs

@@ -0,0 +1,12 @@
+type an_int = int;
+
+fn cmp(x: option<an_int>, y: option<int>) -> bool {
+    x == y
+}
+
+fn main() {
+    assert !cmp(some(3), none);
+    assert !cmp(some(3), some(4));
+    assert cmp(some(3), some(3));
+    assert cmp(none, none);
+}

src/test/run-pass/regions-fn-subtyping.rs

@@ -1,5 +1,3 @@
-//xfail-test
-
 // Issue #2263.
 
 // Should pass region checking.