Emit shorter code and avoid use of a register if the root array is
available.

R=mstarzinger@chromium.org

Bug: v8:6600
Change-Id: I20613af154c8dcc6dc22a81c77fd2faded2dad1e
Reviewed-on: https://chromium-review.googlesource.com/965061Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51986}