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

[Fangrui Song via llvm-commits]

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@@ -314,8 +319,12 @@ template <typename DomTreeT> struct SemiNCAInfo {
 
       // Initialize the semi dominator to point to the parent node.
       WInfo.Semi = WInfo.Parent;
-      for (unsigned N : WInfo.ReverseChildren) {
-        unsigned SemiU = NumToInfo[eval(N, i + 1, EvalStack, NumToInfo)]->Semi;
+      unsigned RCIdx = WInfo.ReverseChildrenStart;
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MaskRay wrote:

Well, I disagree with this. It's more important to drop the confusing sentinel element. List appending actually saves one `-1`.

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