GetUnusedRegister may spill registers and thus modify stack slots.
Therefore, we have to call it before fetching stack slots.
This is another instance of
https://chromium-review.googlesource.com/c/v8/v8/+/3217199.

Bug: v8:7748
Change-Id: I9ff28b26f2dce93ef7b71c1100d9bf88f7c2f7c6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3226327Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77414}