fix universe map in ifcx.instantiate_canonical_*
Previously, `infcx.instantiate_canonical_*` maps the root universe in `canonical` into `ty::UniverseIndex::Root`, I think because it assumes it works with a fresh `infcx` but this is not true for the use cases in mir typeck. Now the root universe is mapped into `infcx.universe()`.
I catched this accidentally while reviewing the code. I'm not sure if this is the right fix or if it is really a bug!
90711a86e5