Fix miscompile due to SSA aliasing

[Stefan J. Wernli (swernli)]

This change updates variable mapping logic in RIR passes to differentiate between "deep" and "shallow" mappings, where the former indicates the mapping should be followed through successive links until the terminator operand is reached and the latter indicates no further mapping should be performed. Fixes #3769

[Andrew Casey (amcasey)]

This doesn't depend on whether it's deep or shallow?

[Stefan J. Wernli (swernli)]

Not directly, but operand.mapped(var_map) will follow the deep vs shallow rules, so stop at the first shallow mapping.

[Andrew Casey (amcasey)]

operation Main() : Int {
    use q0 = Qubit();
    use q1 = Qubit();

    X(q0);
    X(q1);

    mutable n = 0;
    if MResetZ(q0) == One {
        n = 1;
    }

    let h = n;

    if MResetZ(q1) == One {
        n = 2;
    }

    h + n
}

Expected: 3
Actual: 4

Speculative explanation (copilot): At ssa_transform.rs:167, only Shallow(n) aliases are redirected when the earlier phi for n is created. An alias that is still Deep(n) remains tied to the mutable ID. Later, ssa_transform.rs:190 can overwrite the shallow mapping created by a branch-local mutation.

[Stefan J. Wernli (swernli)]

operation Main() : Int {

    use q0 = Qubit();

    use q1 = Qubit();



    X(q0);

    X(q1);



    mutable n = 0;

    if MResetZ(q0) == One {

        n = 1;

    }



    let h = n;



    if MResetZ(q1) == One {

        n = 2;

    }



    h + n

}

Expected: 3

Actual: 4

Speculative explanation (copilot): At ssa_transform.rs:167, only Shallow(n) aliases are redirected when the earlier phi for n is created. An alias that is still Deep(n) remains tied to the mutable ID. Later, ssa_transform.rs:190 can overwrite the shallow mapping created by a branch-local mutation.

This is the resolved in the latest changes.