[Dominators] Improve error checking in deleteEdge

[kuhar]

Summary

This patch improves error detection in deleteEdge. It asserts that the edge doesn't exist in the CFG and that DomTree knew about this edge before.

[kuhar]

Only run the check in debug configuration.

[grosser]

Can you maybe document that splitEdge is supposed to only be called on nodes that are part of the dominator tree.

Also, what about blocks that are unreachable form the entry and not part of the dominator tree?

[kuhar]

@grosser: (I think that you meant to send that in the other review)
Updates to unreachable regions (IE nodes not in the DT) are essentially NOPs, so the logic should also work in that case.

include/llvm/Support/GenericDomTreeConstruction.h

bool verifyReachability(const DomTreeT &DT) {
 
     return true;
   }
 
   static void DeleteEdge(DomTreeT &DT, const NodePtr From, const NodePtr To) {
     assert(From && To && "Cannot disconnect nullptrs");
     DEBUG(dbgs() << "Deleting edge " << BlockNamePrinter(From) << " -> "
                  << BlockNamePrinter(To) << "\n");
+
+#ifndef NDEBUG
+    // Ensure that the edge was in fact deleted from the CFG before informing
+    // the DomTree about it.
+    // The check is O(N), so run it only in debug configuration.
+    auto IsSuccessor = [](const NodePtr SuccCandidate, const NodePtr Of) {
+      auto Successors = ChildrenGetter<NodePtr, IsPostDom>::Get(Of);
+      return llvm::find(Successors, SuccCandidate) != Successors.end();
+    };
+    (void)IsSuccessor;
+    assert(!IsSuccessor(To, From) && "Deleted edge still exists in the CFG!");
+#endif
+
     const TreeNodePtr FromTN = DT.getNode(From);
     // Deletion in an unreachable subtree -- nothing to do.
     if (!FromTN) return;
 
     const TreeNodePtr ToTN = DT.getNode(To);
+    assert(ToTN && "To already unreachable -- there is no edge to delete");
     const NodePtr NCDBlock = DT.findNearestCommonDominator(From, To);
     const TreeNodePtr NCD = DT.getNode(NCDBlock);
 
     // To dominates From -- nothing to do.
     if (ToTN == NCD) return;
 
     const TreeNodePtr ToIDom = ToTN->getIDom();
     DEBUG(dbgs() << "\tNCD " << BlockNamePrinter(NCD) << ", ToIDom "