Empty blocks were skipped when performing register merges (since they
don't have a state), but we were still doing phi value injection only
when visiting the empty block. This meant that empty blocks have
inconsistent register state with the nodes they are trying to use.

Fix this by removing the skipping code, and adding a register merge
state to empty blocks.

Bug: v8:7700
Change-Id: I305b1474c1f956c5c5775c62e3cd1d0c70b5cfdc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3698553Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81129}