In WasmInstanceObject we want to reorder the fields such that fields
that are often accessed from generated code have a small offset (<128
bytes). This requires mixing tagged and untagged fields.
This CL prepares the existing GC support for WasmInstanceObject to only
visit the listed tagged fields.

R=titzer@chromium.org

Bug: chromium:839919
Change-Id: Ia85d7bcfff54af35785b6d573e7e682a26c39ae6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1538119Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60468}