Serialisation roundtrip breaks LoadConst

[aborgna-q]

When running this simple test case that encodes/decodes a simple load const operation,

    #[test]
    fn constants() -> Result<(), Box<dyn std::error::Error>> {
        let mut builder = DFGBuilder::new(FunctionType::new(vec![], vec![FLOAT64_TYPE])).unwrap();
        let w = builder.add_load_const(ConstF64::new(0.5))?;
        let hugr = builder.finish_hugr_with_outputs([w], &FLOAT_OPS_REGISTRY)?;

        let ser = serde_json::to_string(&hugr)?;
        let deser = serde_json::from_str(&ser)?;
        assert_eq!(hugr, deser);
        Ok(())
    }

the extracted edge between the const definition and load gets an invalid target.

See the dot render:

Original	Decoded

This seems to be due to some inconsistent logic in the encoder/decoder, related to #777.