Rollup merge of #114829 - compiler-errors:next-solver-only-unsize-to-dyn-once, r=lcnr

Separate `consider_unsize_to_dyn_candidate` from other unsize candidates

Move the unsize candidate assembly *just for* `T -> dyn Trait` out of `assemble_candidates_via_self_ty` so that we only consider it once, instead of for every normalization step of the self ty. This makes sure that we don't assemble several candidates that are equal modulo normalization when we really don't care about normalizing the self type of an `T: Unsize<dyn Trait>` goal anyways.

Fixes rust-lang/trait-system-refactor-initiative#57

r? lcnr

Author: GuillaumeGomez