Ridable mobs aren't knocked back upon death

Relates to MCPE-19222

This is very likely caused by the fix of MC-109954. 

Can confirm in 21w06a.

can confirm in 21w07a

The mobs listed above all override the method LivingEntity::travel which controls the movement logic for mobs and players alike. Horses, donkeys, mules, llamas, trader llamas, zombie horses and skeleton horses all extend the same base class called AbstractHorse. Pigs and striders implement the interface ItemSteerable which handles its movement when ridden while its rider is holding the needed item for steering.

In the beginning of this method, these mobs checks if it is alive, which means all motion is skipped when it is dead. This check was likely put to stop these mobs moving when dying while being ridden. This check should be removed from the travel method and instead the method LivingEntity::isImmobile should be overridden which if true stop all AI logic for mobs. By default this method returns LivingEntity::isDeadOrDying. It seems this was attempted in the AbstractHorse class, however there is a simple mistake. Instead of this:

protected boolean isImmobile() {
    return super.isImmobile() && this.isVehicle() && this.isSaddled() || this.isEating() || this.isStanding();
 }

The logical AND should be replaced with logical OR after super.isImmobile(), like this:

protected boolean isImmobile() {
    return super.isImmobile() || this.isVehicle() && this.isSaddled() || this.isEating() || this.isStanding();
 }

The above code should be applied to pigs and striders as well without the specific horse methods.

Can confirm in 1.17.1.

Can confirm in 1.18.2 and 22w13a.

Can confirm in 1.19.

Can confirm in 1.19.2.