A few more fixes for visiting bytecodes that are known to be dead:

  * Change JumpLoop dead frame merging to remove the loop predecessor,
    by moving the loop predecessor to be the last in the list and
    allowing it to be dropped the same way as other predecessors.
  * Remove the bytecode walk in EmitUnconditionalDeopt, opting instead
    to check for null current_block in the real bytecode visitor. This
    allows us to handle the case where the start of a basic block is
    dead, but there's no fallthrough into it, so it wouldn't be visited
    by the loop in EmitUnconditionalDeopt.

Bug: v8:7700
Change-Id: I7cf1a54c49a2affc0363c1a0919bb3d427f83f5c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3700070
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81109}