Rollup merge of #122172 - compiler-errors:rpitit-collect-ice, r=fmease

Don't ICE if we collect no RPITITs unless there are no unification errors

Move an assertion in `collect_return_position_impl_trait_in_trait_tys` to after the `ObligationCtxt::eq` calls, so that we only assert and ICE if we have unification errors.

Fixes #121468

Author: matthiaskrgr