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

[Fangrui Song via llvm-commits]

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@@ -753,15 +774,23 @@ class ExtTSPImpl {
             BestGain = CurGain;
             BestChainPred = ChainPred;
             BestChainSucc = ChainSucc;
+            // Stop early when the merge is as good as the previous one.
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MaskRay wrote:

If PrevScore is changed to a std::optional, add an `assert(!PrevScore || BestGain.score() > *PrevScore + EPS)`

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