Mutable variables in dynamic branches prevent full constant folding in partial evaluation

[idavis]

Given a mutable variable used in a quantum condition block, partial eval can't track that despite value being in a dynamic condition, it is statically constant.

@EntryPoint()
operation Test() : Unit {
    mutable value = 0;
    use q = Qubit();
    let cond = MResetZ(q);
    if cond == Zero {
        value -= 1 ;
    } else {
        value += 1;
    }
}

This leads to a variable being used in the CFG for the else block, which is converted into a constant during SSA transformation. This leaves SSA instructions performing mathematical computation on constant values.

This currently causes a panic in the compiler which assumes all constant operations were resolved during partial evaluation.

Actual:

Blocks:
Block 0:Block:
    Variable(0, Integer) = Store Integer(0)
    Call id(1), args( Qubit(0), Result(0), )
    Variable(1, Boolean) = Call id(2), args( Result(0), )
    Variable(2, Boolean) = Icmp Eq, Variable(1, Boolean), Bool(false)
    Branch Variable(2, Boolean), 2, 3
Block 1:Block:
    Call id(3), args( Integer(0), Pointer, )
    Return
Block 2:Block:
    Variable(0, Integer) = Store Integer(-1)
    Jump(1)
Block 3:Block:
    Variable(3, Integer) = Add Variable(0, Integer), Integer(1)
    Variable(0, Integer) = Store Variable(3, Integer)
    Jump(1)

In SSA processing:

Variable(3, Integer) = Add Variable(0, Integer), Integer(1)
Variable(0, Integer) = Store Variable(3, Integer)

Is turned into:

Variable(0, Integer) = Add Integer(0), Integer(1)

Which causes the panic. Given that partial evaluation can't fully perform constant folding, we most likely need to implement more SSA passes for constant folding, dead code elimination, at a minimum.

Workaround:
Save off the current mut value and use it on the rhs of the expressions in the conditional blocks:

operation Workaround() : Unit {
    mutable value = 0;
    use q = Qubit();
    let cond = MResetZ(q);
    let orig = value;
    if cond == Zero {
        value = orig - 1 ;
    } else {
        value = orig + 1;
    }
}