This is similar to the previous SIMD spilling fixes, but this time at
block merges. The logic is similar to the existing cases, but not quite
the same. I did not find a nice way to unify the different locations
where we check for SIMD register overlap.

R=thibaudm@chromium.org

Bug: chromium:1283395, v8:12330
Change-Id: I5ab9b6831368cbce40b8368e4ec7954e985bff96
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3404780Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78720}