QIR Generation panics when conditional branches use early return

[swernli]

The following code causes a panic in QIR generation:

function Branch(value : Bool) : Int {
    if (value) {
        return 3;
    } else {
        return 4;
    }
}
@EntryPoint()
operation Main() : Int {
    use qubits = Qubit[2];
    H(qubits[0]);
    CNOT(qubits[0], qubits[1]);
    let result = Branch(MResetZ(qubits[0]) == Zero);
    result
}

However, the same code with implicit returns in the function works as expected:

function Branch(value : Bool) : Int {
    if (value) {
        3
    } else {
        4
    }
}
@EntryPoint()
operation Main() : Int {
    use qubits = Qubit[2];
    H(qubits[0]);
    CNOT(qubits[0], qubits[1]);
    let result = Branch(MResetZ(qubits[0]) == Zero);
    result
}

This appears to be because of how the BranchControlFlow::Return is used to short circuit the handling of the rest of the block:

qsharp/compiler/qsc_partial_eval/src/lib.rs

Lines 1837 to 1839 in cd5c2e3

	if body_control.is_return() {
	return Ok(BranchControlFlow::Return(body_control.into_value()));
	}

This causes the generated RIR program to be missing branch and jump terminator instructions at the end of blocks. The empty block (should have one terminating jump) is what triggers the specific panic, but other non-empty blocks also are missing terminators that mean the program is invalid.

Either the early return should be fully handled by refactoring the control flow handling for dynamic branches (which must correctly handling inlining for functions like the example above) or the RuntimeFeatureFlags::ReturnWithinDynamicScope should be updated to require more than just TargetCapabilityFlags::Adaptive so that RCA rejects this pattern at compile time.