Simplify insert_outlives_predicate opaque type logic

src/test/ui/type-alias-impl-trait/unbounded_opaque_type.rs

@@ -0,0 +1,8 @@
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+type Opaque<T> = impl Sized;
+fn defining<T>() -> Opaque<T> {}
+struct Ss<'a, T>(&'a Opaque<T>);
+
+fn main() {}