[MemorySSA] Remove dominator umbering

[davide]

Summary

Apparently this was originally used in the phi placement algorithm, but now it's pretty much dead.
I wasn't sure if you planned to use for something else in the future, hence this review.

[• dberlin]

No, this is definitely dead now i believe :)

llvm/trunk/include/llvm/Transforms/Utils/MemorySSA.h

private:
   void optimizeUses();
 
   void verifyUseInDefs(MemoryAccess *, MemoryAccess *) const;
   using AccessMap = DenseMap<const BasicBlock *, std::unique_ptr<AccessList>>;
   using DefsMap = DenseMap<const BasicBlock *, std::unique_ptr<DefsList>>;
 
   void
   determineInsertionPoint(const SmallPtrSetImpl<BasicBlock *> &DefiningBlocks);
-  void computeDomLevels(DenseMap<DomTreeNode *, unsigned> &DomLevels);
   void markUnreachableAsLiveOnEntry(BasicBlock *BB);
   bool dominatesUse(const MemoryAccess *, const MemoryAccess *) const;
   MemoryPhi *createMemoryPhi(BasicBlock *BB);
   MemoryUseOrDef *createNewAccess(Instruction *);
   MemoryAccess *findDominatingDef(BasicBlock *, enum InsertionPlace);
   void placePHINodes(const SmallPtrSetImpl<BasicBlock *> &,
                      const DenseMap<const BasicBlock *, unsigned int> &);
   MemoryAccess *renameBlock(BasicBlock *, MemoryAccess *, bool);

llvm/trunk/lib/Transforms/Utils/MemorySSA.cpp

if (ChildIt == Node->end()) {
       } else
         IncomingVal = renameBlock(BB, IncomingVal, RenameAllUses);
       renameSuccessorPhis(BB, IncomingVal, RenameAllUses);
       WorkStack.push_back({Child, Child->begin(), IncomingVal});
     }
   }
 }
 
-/// \brief Compute dominator levels, used by the phi insertion algorithm above.
-void MemorySSA::computeDomLevels(DenseMap<DomTreeNode *, unsigned> &DomLevels) {
-  for (auto DFI = df_begin(DT->getRootNode()), DFE = df_end(DT->getRootNode());
-       DFI != DFE; ++DFI)
-    DomLevels[*DFI] = DFI.getPathLength() - 1;
-}
-
 /// \brief This handles unreachable block accesses by deleting phi nodes in
 /// unreachable blocks, and marking all other unreachable MemoryAccess's as
 /// being uses of the live on entry definition.
 void MemorySSA::markUnreachableAsLiveOnEntry(BasicBlock *BB) {
   assert(!DT->isReachableFromEntry(BB) &&
          "Reachable block found while handling unreachable blocks");
 
   // Make sure phi nodes in our reachable successors end up with a