The SwitchOnGeneratorState bytecode now also falls through if the
generator object is undefined (so that we don't need that jump) and
restores generator context (so that we don't need that PushContext).
This saves 10 bytes per generator.

Change-Id: Ie0872c827119b9f1d1e9244d3be6496a30cd9620
Reviewed-on: https://chromium-review.googlesource.com/867051
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50845}