Then you can't use pin change interrupts. Only INT0 and INT1 are available as external interrupts on the ATmega8.
Instead of this:
attachPinChangeInterrupt(LEFT_RECEIVE_PIN, IRReceiveLeftISR, FALLING);
attachPinChangeInterrupt(RIGHT_RECEIVE_PIN, IRReceiveRightISR, FALLING);
Use attachInterrupt() and make sure the IR sensors are correctly wired to the INT0 and INT1 pins. Other changes to the code and wiring may be necessary.