[wasm] Simplify access to Isolate in the interpreter.

R=clemensh@chromium.org

Change-Id: I643d9a142e55405ec010cfe56915527e3cff2d6d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1552783
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60632}