[llvm] [Support][NFC] Use single predecessor array in DomTreeConstr (PR #207535)

[Fangrui Song via llvm-commits]

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@@ -78,6 +78,10 @@ template <typename DomTreeT> struct SemiNCAInfo {
                      DenseMap<NodePtr, InfoRec>>
       NodeInfos;
 
+  /// Reverse children of nodes; pairs of (DFSNum (predecessor), next-or-zero);
+  /// forms a linked list in this vector; first entry is sentinel.
+  SmallVector<std::pair<unsigned, unsigned>> ReverseChildren = {{0, 0}};
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MaskRay wrote:

I believe inline capacity 32 helps here per https://llvm-compile-time-tracker.com/compare.php?from=3dc58a6e2c5ad5254883e6a5c43e2e6ea46c42af&to=1bd2dedd34962b737deb0c6c6d3e67a5d6517c46&stat=instructions:u

https://github.com/llvm/llvm-project/pull/207535