[llvm] [SimplifyCFG] Avoid threading for loop headers (PR #151142)

[Arne Stenkrona via llvm-commits]

ArneStenkrona2 wrote:

Thank you both for taking the time to review this, much appreciated.

> Possibly we could still allow threading for cases where it unrolls the loop
That sounds sensible. Another idea I had was to collect a limited number of BBs with e.g. a DFS and see if the whole loop could be identified, which could allow us to check if the threading kept structured control flow. I thought I'd keep the patch as simple as possible though. 

https://github.com/llvm/llvm-project/pull/151142