SimpleRewrite panics on multiports

[aborgna-q]

Given this hugr:

graph LR
    subgraph 0 ["(0) DFG"]
        direction LR
        1["(1) Input"]
        1--"0:0<br>[]+[]"-->3
        2["(2) Output"]
        3["(3) logic.Not"]
        3--"0:0<br>[]+[]"-->4
        3--"0:0<br>[]+[]"-->5
        4["(4) logic.Not"]
        4--"0:0<br>[]+[]"-->2
        5["(5) logic.Not"]
        5--"0:1<br>[]+[]"-->2
    end

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I wrote a simple replacement to replace all the nots with an empty diagram, copying the input bool to the outputs.

This throws an error:

thread 'hugr::rewrite::simple_replace::test::test_copy_inputs' panicked at hugr-core/src/hugr/rewrite/simple_replace.rs:610:92:
called `Option::unwrap()` on a `None` value

see the failing test case here

hugr/hugr-core/src/hugr/rewrite/simple_replace.rs

Line 615 in 6dbdbb0

fn test_copy_inputs(