Add support for i64 loads and stores.

R=ahaas@chromium.org

Bug: v8:6600
Change-Id: I494fb466f702c8b89b96f7ded6d1cf09ec6991fb
Reviewed-on: https://chromium-review.googlesource.com/902046
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51207}