We weren't always clearing latest_checkpoint_state on merge points, so
bottleneck it in a better location.

Bug: v8:7700
Change-Id: Iaac5922d769d97d49b85613d5390196a14ad8059
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3657437Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80691}