Instead of specializing {DecodeLoadMem} and {DecodeStoreMem} for
prefixed opcodes, just pass the offset as parameter.

R=titzer@chromium.org

Change-Id: I0581f6e9cb562885709aaa0317bb550b175ad160
Reviewed-on: https://chromium-review.googlesource.com/822391Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50039}