[maglev] Prepare BranchIfCompare

Define dummy BranchIfCompare, a conditional control node
that accepts more than one input node.

Bug: v8:7700
Change-Id: If29b6e9531e8a75a4e94fb2057d8e8d36cf0d9ee
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3506376


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@{#79399}