This moves constant nodes to separate data structures on the graph so
they can be looked up there. Graph processors walk the constants before
walking other nodes.

Bug: v8:7700
Change-Id: Id4bec2c2a26011dcacf3355fe17d821451f79397
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3706625
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81195}