Unconditional eager deopts from lack of feedback (née soft deopts) mean
that the remainder of the basic block is dead. Avoid emitting this code
by fast forwarding the iterator until the next merge.

The EagerDeopt node becomes a Deopt control node which terminates its
own block (this is to avoid spurious control flow after the EagerDeopt,
or weirdness with liveness). A concept of "merging dead blocks" has to
be introduced so that the successors of the killed block still have the
right number of predecessors.

Bug: v8:7700
Change-Id: Id9c442c3b18d3f394dc2411604d0c8503d6aaae2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3578647Reviewed-by: Jakob Linke <jgruber@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79952}