This is in preparation of making sure that jump table slots don't cross
cache line boundaries. It is only introducing helper functions for back
and forth conversion between "index" and "offset", but should not make
any functional changes yet.

R=ahaas@chromium.org
BUG=v8:8018

Change-Id: I6ab525f9b89a6a15414c043a54c9fffb527a1ab6
Reviewed-on: https://chromium-review.googlesource.com/1163517Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54922}