Add support for loading and storing i32 values to and from memory.
Support for f32 and for non-traphandler configurations (i.e. emitting
bounds checks) will be added in a follow-up CL.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I43c44ce61f5acbac325261212374eb0f48c6ee89
Reviewed-on: https://chromium-review.googlesource.com/809164Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50068}