Explicitly initialize the loop merge's back-edge predecessor to a
specfic "uninitialized" value, distinct from nullptr (which marks dead
loops) and done in both debug and release modes.

Bug: v8:7700
Change-Id: I6a845cc4dbd7da75954f78607e69a5d4e2ec1ec7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3645114Reviewed-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@{#80525}