Instead of computing the jump table slot, and subtracting the start of
the jump table, we can just compute the offset directly. This method is
then used to compute jump table slots.

R=titzer@chromium.org

Change-Id: I634e5ee71a0d2e9d0d2db0ea373f833df91fba8e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1709419Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62849}