A spec change (https://github.com/tc39/proposal-async-iteration/commit/a0dfeba1a8029012b6e964099929b8a157818c9f) introduced a number of Await operations to the spec. In turn, this caused generated bytecode for async generators to grow drastically.

This commit moves the Await within AsyncGeneratorYield (https://tc39.github.io/proposal-async-iteration/#sec-asyncgeneratoryield step 5) into a new TFJ builtin, similar in structure to AsyncGeneratorAwait, but instead of resuming the generator on resolution of the Promise, the current generator request's Promise is fulfilled instead.

This results in a reduction in generated bytecode without losing any statically available information.

BUG=v8:5855

Change-Id: Ib5bcf06132d221beffdea30639a7b4437030143b
Reviewed-on: https://chromium-review.googlesource.com/582487
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47224}