Make match exhaustive.

compiler/rustc_mir_transform/src/promote_consts.rs

@@ -141,7 +141,7 @@ fn visit_local(&mut self, index: Local, context: PlaceContext, location: Locatio
                 }
                 TempState::Unpromotable
             }
-            _ => TempState::Unpromotable,
+            TempState::Unpromotable | TempState::PromotedOut => TempState::Unpromotable,
         };
     }