Consistency of `show_dot_graph` and `get_num_nodes`

A question came up while revisiting #489 with @kajalpatelinfo: should `show_dot_graph` be consistent with `get_num_nodes`, in the sense that the number of nodes displayed in the graph matches what `get_num_nodes` computes? It looks like `show_dot_graph`'s code is based on `CopyMapper`, which ignores duplicates, but currently `get_num_nodes` uses `id(expr)` for a hash key, meaning it counts duplicates separately.

I'm tempted to think they should behave the same, at least by default. It might actually be useful to give them both an option to include/exclude duplicates? Thoughts @inducer?