Deduplicate unreachable blocks, for real this time In https://github.com/rust-lang/rust/pull/106428 (in particular https://github.com/rust-lang/rust/pull/106428/commits/41eda69516dd3ee217ae07c0efa369d31f630405) we noticed that inlining `unreachable_unchecked` can produce duplicate unreachable blocks. So we improved two MIR optimizations: `SimplifyCfg` was given a simplify to deduplicate unreachable blocks, then `InstCombine` was given a combiner to deduplicate switch targets that point at the same block. The problem is that change doesn't actually work. Our current pass order is ``` SimplifyCfg (does nothing relevant to this situation) Inline (produces multiple unreachable blocks) InstCombine (doesn't do anything here, oops) SimplifyCfg (produces the duplicate SwitchTargets that InstCombine is looking for) ``` So in here, I have factored out the specific function from `InstCombine` and placed it inside the simplify that produces the case it is looking for. This should ensure that it runs in the scenario it was designed for. Fixes https://github.com/rust-lang/rust/issues/110551 r? `@cjgillot`