[DomTree] Remove unnecessary domtree level check in SemiNCA (NFC) (#73107)

runSemiNCA() currently checks that the ReverseChildren are below
MinLevel in the DT, which is used when performing incremental updates.

However, ReverseChildren is populated during runDFS with only the
predecessors that are part of that DFS walk, which will itself be level
limited in the relevant cases. As such, I don't believe that this should
be checked during runSemiNCA().

This code probably dates back to a time when predecessors were not
cached during runDFS and as such not limited to the visited subtree
only.

Author: nikic

llvm/include/llvm/Support/GenericDomTreeConstruction.h

@@ -268,7 +268,7 @@ struct SemiNCAInfo {
   }
 
   // This function requires DFS to be run before calling it.
-  void runSemiNCA(DomTreeT &DT, const unsigned MinLevel = 0) {
+  void runSemiNCA() {
     const unsigned NextDFSNum(NumToNode.size());
     SmallVector<InfoRec *, 8> NumToInfo = {nullptr};
     NumToInfo.reserve(NextDFSNum);
@@ -291,11 +291,6 @@ struct SemiNCAInfo {
         assert(NodeToInfo.contains(N) &&
                "ReverseChildren should not contain unreachable predecessors");
 
-        const TreeNodePtr TN = DT.getNode(N);
-        // Skip predecessors whose level is above the subtree we are processing.
-        if (TN && TN->getLevel() < MinLevel)
-          continue;
-
         unsigned SemiU = NumToInfo[eval(N, i + 1, EvalStack, NumToInfo)]->Semi;
         if (SemiU < WInfo.Semi) WInfo.Semi = SemiU;
       }
@@ -576,7 +571,7 @@ struct SemiNCAInfo {
     DT.Roots = FindRoots(DT, PostViewBUI);
     SNCA.doFullDFSWalk(DT, AlwaysDescend);
 
-    SNCA.runSemiNCA(DT);
+    SNCA.runSemiNCA();
     if (BUI) {
       BUI->IsRecalculated = true;
       LLVM_DEBUG(
@@ -904,7 +899,7 @@ struct SemiNCAInfo {
 
     SemiNCAInfo SNCA(BUI);
     SNCA.runDFS(Root, 0, UnreachableDescender, 0);
-    SNCA.runSemiNCA(DT);
+    SNCA.runSemiNCA();
     SNCA.attachNewSubtree(DT, Incoming);
 
     LLVM_DEBUG(dbgs() << "After adding unreachable nodes\n");
@@ -998,7 +993,7 @@ struct SemiNCAInfo {
     SemiNCAInfo SNCA(BUI);
     SNCA.runDFS(ToIDom, 0, DescendBelow, 0);
     LLVM_DEBUG(dbgs() << "\tRunning Semi-NCA\n");
-    SNCA.runSemiNCA(DT, Level);
+    SNCA.runSemiNCA();
     SNCA.reattachExistingSubtree(DT, PrevIDomSubTree);
   }
 
@@ -1122,7 +1117,7 @@ struct SemiNCAInfo {
                       << BlockNamePrinter(PrevIDom) << "\nRunning Semi-NCA\n");
 
     // Rebuild the remaining part of affected subtree.
-    SNCA.runSemiNCA(DT, MinLevel);
+    SNCA.runSemiNCA();
     SNCA.reattachExistingSubtree(DT, PrevIDom);
   }