Add a method which optionally merges dead fallthrough paths, in case the
iteration in EmitUnconditionalDeopt reaches a merge point that is live
from another jump but dead on the fallthrough.

Bug: v8:7700
Change-Id: Ie505cd5356fcf70208f2f6d3e52b805956485f74
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3663086
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80878}