[llvm] [DTU] fix dominator tree update eliding reachable nodes (PR #177683)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

> [...]
>
> Have you tried running with expensive checks on some real-world code? This will enable the full verifier.

​Sorry for the delay - I started it and (not suprisingly) it took a while, then got side-tracked.

TL;DR; self-hosting clang worked fine.

Detail: at `418ba6e8ae2cde7924388142b8ab90c636d2c21f` + this change, I did:

`cmake ../llvm -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS="clang"  -GNinja  -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_ENABLE_EXPENSIVE_CHECKS=ON`

`ninja check-all`

then used that ninja to build clang:

`cmake ../llvm -GNinja -DCMAKE_C_COMPILER=/work/llvm-project/expchecks/bin/clang -DCMAKE_CXX_COMPILER=/work/llvm-project/expchecks/bin/clang++`

https://github.com/llvm/llvm-project/pull/177683