Linearity checking bug

[mark-koch]

The following program incorrectly fails linearity checking:

@guppy.struct(module)
class MyStruct:
    q: qubit


@guppy.declare(module)
def use(q: qubit) -> None: ...


@guppy(module)
def test(s: MyStruct) -> MyStruct:
    use(s.q)
    while True:
        s.q = qubit()
        return s
    # Guppy is not yet smart enough to detect that this code is unreachable
    s.q = qubit()
    return s