perf(internal): optimize Subscription#add() for the common case (#4489)

This removes unnecessary checks on the `teardown` parameter upfront, and
instead moves these checks into the appropriate branches guarded by the
type check. This way we get the same behavior, but with fewer checks and
smaller code size.

Also optimize adding the child `subscription` to `this._subscriptions`
for the common case that there were no previous subscriptions.

src/internal/Subscription.ts

@@ -143,21 +143,13 @@ export class Subscription implements SubscriptionLike {
    * list.
    */
   add(teardown: TeardownLogic): Subscription {
-    if (!teardown || (teardown === Subscription.EMPTY)) {
-      return Subscription.EMPTY;
-    }
-
-    if (teardown === this) {
-      return this;
-    }
-
-    let subscription = (<Subscription> teardown);
-
+    let subscription = (<Subscription>teardown);
     switch (typeof teardown) {
       case 'function':
-        subscription = new Subscription(<(() => void) > teardown);
+        subscription = new Subscription(<(() => void)>teardown);
       case 'object':
-        if (subscription.closed || typeof subscription.unsubscribe !== 'function') {
+        if (subscription === this || subscription.closed || typeof subscription.unsubscribe !== 'function') {
+          // This also covers the case where `subscription` is `Subscription.EMPTY`, which is always in `closed` state.
           return subscription;
         } else if (this.closed) {
           subscription.unsubscribe();
@@ -168,13 +160,21 @@ export class Subscription implements SubscriptionLike {
           subscription._subscriptions = [tmp];
         }
         break;
-      default:
+      default: {
+        if (!(<any>teardown)) {
+          return Subscription.EMPTY;
+        }
         throw new Error('unrecognized teardown ' + teardown + ' added to Subscription.');
+      }
     }
 
-    const subscriptions = this._subscriptions || (this._subscriptions = []);
-
-    subscriptions.push(subscription);
+    // Optimize for the common case when adding the first subscription.
+    const subscriptions = this._subscriptions;
+    if (subscriptions) {
+      subscriptions.push(subscription);
+    } else {
+      this._subscriptions = [subscription];
+    }
     subscription._addParent(this);
 
     return subscription;