Change Checkpoints from IR Nodes to just normal Zone objects, pointed to
by the deopting Node. Also merge Checkpoint and DeoptimizationInfo --
this has the side effect that multiple Nodes that share a checkpoint
will point to the exact same deopt call.

Bug: v8:7700
Change-Id: Ib36aa13afe3af6a0a22d2cfe80a13fef4bea1227
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3545179
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79607}