Armored entities from pre-1.9 worlds upgrade to dual wielding armor

When updating mobs that are wearing armor in version 1.8.9 and before, directly to 18w19a and after, all mobs swap armor slots with hand slots, resulting in the mobs wearing items, and holding armor. This can also cause skeletons, wither skeletons, and zombie pigmen/piglins to be empty handed. 

Steps to reproduce:

1. Spawn a mob with armor (only boots and leggings work) in any version before 15w31a
2. Update the world directly to any version after 18w16a
3. All mobs that were previously wearing armor should now be dual wielding leggings and boots

Here is a list of all affected mobs:
Zombie
Baby Zombie
Zombie Villager
Baby Zombie Villager
Zombie Pigman
Skeleton
Wither Skeleton
Armor Stands
Villager (Not Visually Noticeable)