Refactor NodeLabel from GNode, which saves memory and is a step towards
decoupling the node layout from the graph structure.

Change-Id: I095a2f7a7ab28067161deffbc37952ae15410e0a
Notry: true
Bug: v8:7327
Reviewed-on: https://chromium-review.googlesource.com/c/1396418
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58571}