Fix merging of detachedness state.

Drive-by: Split lookup and merging of nodes.

Bug: chromium:1110816
Change-Id: I27dba7a6f22c75e5aae130d8cec01ccf755fad79
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2416492
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69985}