Introduce a new SwitchSmiTable bytecode for generators, which does a
table lookup for the accumulator value in a jump table stored in the
constant array pool. This removes the if-else chains at resumable
function/loop headers.

As a drive-by, add a scoped environment saving struct to the bytecode
graph builder.

Bug: v8:6351
Bug: v8:6366
Change-Id: I63be15a8b599d6684c7df19dedb8860562678fb0
Reviewed-on: https://chromium-review.googlesource.com/500271
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45314}