These enums capture the "type" of a load and store in one value, with
accessors to get the value type, memory type, memory size etc.
This will simplify implementing memory operations in Liftoff.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Ief673e90fe63292853f97e5e95b5c39da1296730
Reviewed-on: https://chromium-review.googlesource.com/822476
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50054}