Auto merge of #121211 - lcnr:nll-relate-handle-infer, r=BoxyUwU

deduplicate infer var instantiation

Having 3 separate implementations of one of the most subtle parts of our type system is not a good strategy if we want to maintain a sound type system ✨ while working on this I already found some subtle bugs in the existing code, so that's awesome 🎉 cc #121159

This was necessary as I am not confident in my nll changes in #119106, so I am first cleaning this up in a separate PR.

r? `@BoxyUwU`

Author: bors