Simplify the model for generating Awaits, because the resume point is
always immediately following the suspend point, and registers used are
always the same for both operations.

Includes a minor refactoring of BytecodeGenerator::VisitYield() to
perform iterator result creation before the SuspendGenerator bytecode,
rather than between SuspendGenerator and Return. This adds a small
number of bytecodes for each yield.

BUG=v8:2355, v8:5855

Change-Id: I4868b89a6bc1b251f887d2a45890c8fa19f7b089
Reviewed-on: https://chromium-review.googlesource.com/576286Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#46820}