[llvm] [LoopSimplifyCFG] Add check for missing loop preheader (PR #149743)

[Justus Klausecker via llvm-commits]

Justus2308 wrote:

I rebased on top of the latest commit with passing ci, hopefully this will fix the test failures. I don't really know what I'm looking at in the logs, some lldb tests are triggering an assertion in `llvm/include/llvm/Support/YAMLTraits.h` that seems to be unrelated to my changes. If this doesn't work I'll ask for help on discord.

https://github.com/llvm/llvm-project/pull/149743