https://github.com/tc39/proposal-async-iteration/commit/e3246ad69cc6f83b34bdd3451c3c6abce37fd1f3
removed some redundancies in yield and yield*.

In particular:
- AsyncGeneratorRawYield becomes unnecessary, and is deleted in this CL
- Parser::RewriteYieldStar() is updated to perform the IteratorValue() algorithm as appropriate

BUG=v8:6187, v8:5855
R=rmcilroy@chromium.org, adamk@chromium.org, littledan@chromium.org, vogelheim@chromium.org

Change-Id: I05e8429b9cbd4531c330ee53a05656b90162064c
Reviewed-on: https://chromium-review.googlesource.com/471806Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#44649}