Adopt graph-based IR in the distant future

[ngernest]

See notes from 12/9 meeting in the meeting notes doc and the whiteboard sketch of the Rust pseudocode below:

Note: this is more so for follow-up work (after March)

[ngernest]

Engineering suggestion from Adrian for when we implement the graph-based IR:

Super tiny engineering suggestion: where you have HashMap<NodeID, Node>, that is an opportunity for flattening, if you want. As in, that could just be a Vec<Node> and the ID would just be a usize, or this could be a cranelift-entity PrimaryMap.

(We can just use PrimaryMap since we already use it elsewhere in our codebase)