[maglev] Snapshot VOs in merge points

This allows setting/storing fields in the same basic block
without copying the VO.

Bug: v8:7700
Change-Id: I9ef2d0e5de39a9fa235b8dc95b5144ef3cab75f5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/5701106


Auto-Submit: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#95123}