make NOP dyn casts not require anything about the vtable

Author: RalfJung

Diff for: src/unsize.rs

@@ -29,6 +29,7 @@ pub(crate) fn unsized_info<'tcx>(
             let old_info =
                 old_info.expect("unsized_info: missing old info for trait upcasting coercion");
             if data_a.principal_def_id() == data_b.principal_def_id() {
+                // A NOP cast that doesn't actually change anything, should be allowed even with invalid vtables.
                 return old_info;
             }