GetUnusedRegister may spill registers and thus modify stack slots.
Therefore, we have to call it before fetching stack slots.

Bug: v8:7748
Change-Id: If7873efd986d9a7a6869fa8ec156d45affd7a8aa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3217199
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77334}