Correctly account for niche-optimized tags in rustc_transmute
This is a bit hacky, but it fixes the ICE and makes it possible to run the safe transmute check on every `mem::transmute` check we instantiate. I want to write a lint that needs to do that, but this stands well on its own.
cc `@jswrenn` here's the fix I alluded to yesterday :)
Fixes#123693
9ed667f8ed