This CL splits out a Graph class from the GraphView, which improves
maintainability and is a first step towards preserving node positions
during phase view changes.

This CL also removes duplication of node storage on the graph and
provides a generator function instead. The only storage for nodes
in the graph is now the {nodeMap}.

Bug: v8:7327
Notry: true
Change-Id: I1659ecfe46f62a12d2fb3c40ccd6f4936f081b53
Reviewed-on: https://chromium-review.googlesource.com/c/1396087
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58549}