For async generators that yield primitives, there's not really a point
in creating temporary promises first, just to immediately fulfill them
with the primitive values passed to `yield`. Instead we can skip those
steps and just directly schedule a PromiseFulfillReactionJobTask to do
the job.

This improves the execution time of fibonacci-async-es2017-native from
around 189ms to roughly 183ms, which corresponds to almost a ~3% boost
here.

Bug: v8:7253
Change-Id: I91413d2cc5ffdc6c851f72f91ff5c207e048c19d
Reviewed-on: https://chromium-review.googlesource.com/966607
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52016}