This adds support for the backing store of mapped arguments objects to
escape analysis. It also unifies two simplified operators representing
allocations of these backing stores into a single {NewArgumentsElements}
operator and threads through the "mapped count" to the deoptimizer.

R=tebbi@chromium.org

Change-Id: I1864e29a5703348597b7b2e41deaf5fab73e2c93
Reviewed-on: https://chromium-review.googlesource.com/643208
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47800}