[builtins] Refactor reaction job allocation in PerformPromiseThen.

Reduce the code duplication overhead in the InternalPerformPromiseThen
helper, which saves quite a bit of space and makes code more concise and
readable.

Bug: v8:7253, v8:7310
Change-Id: I64d11661d7258ced32df564d2e83c5ea45955415
Reviewed-on: https://chromium-review.googlesource.com/880722Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50800}

src/builtins/builtins-promise-gen.cc

@@ -487,36 +487,33 @@ Node* PromiseBuiltinsAssembler::InternalPerformPromiseThen(
 
     BIND(&fulfilled_check);
     {
-      Label reject(this);
-      Node* const result = LoadObjectField(promise, JSPromise::kResultOffset);
-      GotoIfNot(IsPromiseStatus(status, v8::Promise::kFulfilled), &reject);
+      Label fulfilled(this), rejected(this, Label::kDeferred), enqueue(this);
+      VARIABLE(var_tasks, MachineRepresentation::kTagged);
+      Branch(IsPromiseStatus(status, v8::Promise::kFulfilled), &fulfilled,
+             &rejected);
 
-      Node* info = AllocatePromiseReactionJobInfo(
-          result, var_on_resolve.value(), deferred_promise, deferred_on_resolve,
-          deferred_on_reject, context);
-      CallBuiltin(Builtins::kEnqueueMicrotask, NoContextConstant(), info);
-      Goto(&out);
+      BIND(&fulfilled);
+      {
+        var_tasks.Bind(var_on_resolve.value());
+        Goto(&enqueue);
+      }
 
-      BIND(&reject);
+      BIND(&rejected);
       {
         CSA_ASSERT(this, IsPromiseStatus(status, v8::Promise::kRejected));
-        Node* const has_handler = PromiseHasHandler(promise);
-        Label enqueue(this);
-
-        // TODO(gsathya): Fold these runtime calls and move to TF.
-        GotoIf(has_handler, &enqueue);
+        var_tasks.Bind(var_on_reject.value());
+        GotoIf(PromiseHasHandler(promise), &enqueue);
         CallRuntime(Runtime::kPromiseRevokeReject, context, promise);
         Goto(&enqueue);
-
-        BIND(&enqueue);
-        {
-          Node* info = AllocatePromiseReactionJobInfo(
-              result, var_on_reject.value(), deferred_promise,
-              deferred_on_resolve, deferred_on_reject, context);
-          CallBuiltin(Builtins::kEnqueueMicrotask, NoContextConstant(), info);
-          Goto(&out);
-        }
       }
+
+      BIND(&enqueue);
+      Node* result = LoadObjectField(promise, JSPromise::kResultOffset);
+      Node* info = AllocatePromiseReactionJobInfo(
+          result, var_tasks.value(), deferred_promise, deferred_on_resolve,
+          deferred_on_reject, context);
+      CallBuiltin(Builtins::kEnqueueMicrotask, NoContextConstant(), info);
+      Goto(&out);
     }
   }