This adds support for i64 addition and subtraction.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: If7ed762091b0ebd688eb2a8cac84e59b91c8a322
Reviewed-on: https://chromium-review.googlesource.com/992316Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52351}