Improve MaybeStorageLive::initialize_start_block.

We can union the two sets the easy way. This removes the need for the
domain size check, because `union` does that same check itself.

Author: nnethercote

compiler/rustc_mir_dataflow/src/impls/storage_liveness.rs

@@ -28,10 +28,7 @@ impl<'a, 'tcx> Analysis<'tcx> for MaybeStorageLive<'a> {
     }
 
     fn initialize_start_block(&self, body: &Body<'tcx>, on_entry: &mut Self::Domain) {
-        assert_eq!(body.local_decls.len(), self.always_live_locals.domain_size());
-        for local in self.always_live_locals.iter() {
-            on_entry.insert(local);
-        }
+        on_entry.union(&*self.always_live_locals);
 
         for arg in body.args_iter() {
             on_entry.insert(arg);