and Relaxed_Store(int, ...) by migrating the only call site
to using slot increment/decrement instead of offset calculations.
Also use SlotBase::location() more consistently.

Bug: v8:8238
Change-Id: I3099884a2a9e05041114205e7fb81691261afe19
Reviewed-on: https://chromium-review.googlesource.com/c/1349731Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57852}