The method SetInstanceDescriptors accessed the bit field before it got
initialized, which is undefined behavior.

R=cbruni@chromium.org

Change-Id: Ie17e6e840a9a4278e066278d1ce81ac4b836a429
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3400970Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78684}