PIL optimizer fails with Symbol not found

[georgwiese]

To reproduce: Get block_to_block_with_bus.pil and run:

$ cargo run pil output/block_to_block_with_bus.pil -o output -f         
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.18s
     Running `target/debug/powdr pil output/block_to_block_with_bus.pil -o output -f`
Analyzing PIL and computing constraints...
Writing output/block_to_block_with_bus_analyzed.pil.
done.
Optimizing pil...
thread 'main' panicked at /Users/georg/coding/powdr/pilopt/src/lib.rs:94:13:
Symbol not found: main_arith::sel[0]
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I tried debugging this a little bit:

• The problem appears to be this block, which adds a hint. Commenting this out makes the bug disappear.
• collect_required_symbols appears to return main_arith::sel[0], which in my understanding is wrong. It should return main_arith::sel instead.
• main_arith::sel[0] does not have a definition (only the array main_arith::sel)