[llvm] [CodeLayout] Faster basic block reordering, ext-tsp (PR #68617)

[via llvm-commits]

spupyrev wrote:

After more thorough testing, I had to bring back `ChainSplitThreshold` and modify the tests accordingly as otherwise some tests were up to 4x slower. Having this threshold regresses the quality a little bit, but I don't see a clean way.
Adjusted the numbers on my benchmarks to reflect the change.

https://github.com/llvm/llvm-project/pull/68617