[Dominators] Add DomTreeUpdater constructor from DT* and PDT*

[NutshellySima]

Summary

Previously, if a function accepts an optional DT pointer,

void Foo (.., DominatorTree * DT = nullptr) {
  ...
  if(DT)
    DomTreeUpdater(*DT, ...).insertEdge(A, B);
  if(DT){
    DomTreeUpdater DTU(*DT, ...);
    ... // Construct the update vector and applyUpdates
  }
  ...
  if(DT){
    DomTreeUpdater DTU(*DT, ...);
    ... // Construct the update vector and applyUpdates
  }
}

After this patch, it can be simplified as

void Foo (.., DominatorTree * DT = nullptr) {
  DomTreeUpdater DTU(DT, ...);
  ...
  DTU.insertEdge(A, B);
  if(DT){
    ... // Construct the update vector and applyUpdates
  }
  ...
  if(DT){
    ... // Construct the update vector and applyUpdates
  }
}

[kuhar]

LGTM. Should make life easier.

include/llvm/IR/DomTreeUpdater.h

 namespace llvm {
 class DomTreeUpdater {
 public:
   enum class UpdateStrategy : unsigned char { Eager = 0, Lazy = 1 };
 
   explicit DomTreeUpdater(UpdateStrategy Strategy_) : Strategy(Strategy_) {}
   DomTreeUpdater(DominatorTree &DT_, UpdateStrategy Strategy_)
       : DT(&DT_), Strategy(Strategy_) {}
+  DomTreeUpdater(DominatorTree *DT_, UpdateStrategy Strategy_)
+      : DT(DT_), Strategy(Strategy_) {}
   DomTreeUpdater(PostDominatorTree &PDT_, UpdateStrategy Strategy_)
       : PDT(&PDT_), Strategy(Strategy_) {}
+  DomTreeUpdater(PostDominatorTree *PDT_, UpdateStrategy Strategy_)
+      : PDT(PDT_), Strategy(Strategy_) {}
   DomTreeUpdater(DominatorTree &DT_, PostDominatorTree &PDT_,
                  UpdateStrategy Strategy_)
       : DT(&DT_), PDT(&PDT_), Strategy(Strategy_) {}
+  DomTreeUpdater(DominatorTree *DT_, PostDominatorTree *PDT_,
+                 UpdateStrategy Strategy_)
+      : DT(DT_), PDT(PDT_), Strategy(Strategy_) {}
 
   ~DomTreeUpdater() { flush(); }
 
   /// Returns the UpdateStrategy of the class instance.
   UpdateStrategy getUpdateStrategy() const { return Strategy; };
 
   /// Returns true if it holds a DominatorTree.
   bool hasDomTree() const { return DT != nullptr; }

unittests/IR/DomTreeUpdaterTest.cpp

StringRef ModuleString = R"(
                            )";
   // Make the module.
   LLVMContext Context;
   std::unique_ptr<Module> M = makeLLVMModule(Context, ModuleString);
   Function *F = M->getFunction(FuncName);
 
   // Make the DTU.
   DominatorTree DT(*F);
-  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
+  PostDominatorTree *PDT = nullptr;
+  DomTreeUpdater DTU(&DT, PDT, DomTreeUpdater::UpdateStrategy::Lazy);
   ASSERT_TRUE(DTU.hasDomTree());
   ASSERT_FALSE(DTU.hasPostDomTree());
   ASSERT_EQ(DTU.getUpdateStrategy(), DomTreeUpdater::UpdateStrategy::Lazy);
   ASSERT_TRUE(DTU.getDomTree().verify());
 
   Function::iterator FI = F->begin();
   BasicBlock *BB0 = &*FI++;
   BasicBlock *BB1 = &*FI++;
StringRef ModuleString = R"(
                            )";
   // Make the module.
   LLVMContext Context;
   std::unique_ptr<Module> M = makeLLVMModule(Context, ModuleString);
   Function *F = M->getFunction(FuncName);
 
   // Make the DTU.
   DominatorTree DT(*F);
-  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
+  PostDominatorTree *PDT = nullptr;
+  DomTreeUpdater DTU(&DT, PDT, DomTreeUpdater::UpdateStrategy::Lazy);
   ASSERT_TRUE(DTU.hasDomTree());
   ASSERT_FALSE(DTU.hasPostDomTree());
   ASSERT_EQ(DTU.getUpdateStrategy(), DomTreeUpdater::UpdateStrategy::Lazy);
   ASSERT_TRUE(DTU.getDomTree().verify());
 
   Function::iterator FI = F->begin();
   BasicBlock *BB0 = &*FI++;
   BasicBlock *BB1 = &*FI++;
TEST(DomTreeUpdater, LazyUpdateBasicOperations) {
   // Make the module.
   LLVMContext Context;
   std::unique_ptr<Module> M = makeLLVMModule(Context, ModuleString);
   Function *F = M->getFunction(FuncName);
 
   // Make the DTU.
   DominatorTree DT(*F);
   PostDominatorTree PDT(*F);
-  DomTreeUpdater DTU(DT, PDT, DomTreeUpdater::UpdateStrategy::Lazy);
+  DomTreeUpdater DTU(&DT, &PDT, DomTreeUpdater::UpdateStrategy::Lazy);
   ASSERT_TRUE(DTU.hasDomTree());
   ASSERT_TRUE(DTU.hasPostDomTree());
   ASSERT_EQ(DTU.getUpdateStrategy(), DomTreeUpdater::UpdateStrategy::Lazy);
   ASSERT_TRUE(DTU.getDomTree().verify());
   ASSERT_TRUE(DTU.getPostDomTree().verify());
 
   Function::iterator FI = F->begin();
   BasicBlock *BB0 = &*FI++;