PIO `mov invert` can result in a negative value

[urish]

e.g. mov x, ~null will set the X register to -1. This can be an issue when comparing X with Y to check for 0xffffffff value, as the following test case (which passes on silicone) demonstrates:

  it('should correctly compare X to 0xffffffff after executing a `mov x, ~null` instruction', async () => {
    await resetStateMachines();
    await cpu.writeUint32(TXF0, 0xffffffff);
    await cpu.writeUint32(SM0_INSTR, pioPULL(false, false));
    await cpu.writeUint32(SM0_INSTR, pioMOV(PIO_DEST_X, PIO_OP_INVERT, PIO_SRC_NULL)); // X <- ~0 = 0xffffffff
    await cpu.writeUint32(SM0_INSTR, pioOUT(PIO_DEST_Y, 32)); // Y <- 0xffffffff
    await cpu.writeUint32(SM0_INSTR, pioJMP(PIO_COND_ALWAYS, 8));
    await cpu.writeUint32(SM0_INSTR, pioJMP(PIO_COND_XNEY, 16)); // Shouldn't take the jump
    expect(await cpu.readUint32(SM0_ADDR)).toEqual(8); // Assert that the 2nd jump wasn't taken
  });