Auto merge of #144732 - lcnr:ignore-shadowed-impls, r=compiler-errors

dont assemble shadowed impl candidates

Fixes rust-lang/trait-system-refactor-initiative#109.

I've originally intended to fix this by supporting lazy reevaluation when rerunning cycles. This ended up being really difficult, see https://github.com/lcnr/search_graph for my notes used while working on this. It is also insufficient for the `rayon-hang-2.rs` test as we end up with goals which we need to rerun for all combinations of provisional results. While landing such an optimization in the future may still be desirable, it is very difficult and insufficient to fix these hangs. Also see the relevant [zulip thread](https://rust-lang.zulipchat.com/#narrow/channel/364551-t-types.2Ftrait-system-refactor/topic/rustc-rayon.20hang/near/527850058).

I was previously opposed to avoiding assembling shadowed impls as it may prevent future improvements in this area, cc rust-lang/rust#141226. Going to track this and the reasoning behind it in rust-lang/trait-system-refactor-initiative#226.

r? `@BoxyUwU` `@compiler-errors`

Author: bors