Set Instant Pot to saute. Add the bacon pieces and cook until semi-crispy. Remove bacon from pot but leave the grease. Add the butter to the pot and allow to melt. Add garlic and onion and saute for 3 minutes, or until soft and fragrant. Add carrots, celery, potatoes, salt and pepper and give the mixture a stir.
Add chicken stock or broth and press the potatoes/veggies down so they are fully immersed in liquid. Place the cover on the pot and set timer to 10 minutes on high pressure (pressure cook setting). It will take the IP around 20 minutes to build up pressure before it begins cooking. When cooking is done, let pressure release naturally for 5 minutes. Do a quick release to get rid of remaining pressure.
While the pressure is releasing, whisk together 1/2 cup of the milk with the corn starch until lumps are gone. Whisk in remaining milk.
Remove cover and add the milk and cream cheese. Stir until melted.
For a smoother consistency, use an immersion blender to puree the mixture. For a chunkier consistency, serve as-is. Ladle into serving bowls and top with cheddar cheese, reserved bacon and green onions.