This makes temporary variables nestable and fixes borked nesting with
function table calls by introducing a {TemporaryVariableScope} helper.

R=clemensh@chromium.org
TEST=mjsunit/regress/regress-6196
BUG=v8:6196

Change-Id: Ie760f27ce9ede3d4d5dacdebdc295c56cc666970
Reviewed-on: https://chromium-review.googlesource.com/467327
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44367}