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

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
  }
}
```
Patch by Chijun Sima <simachijun@gmail.com>.

Reviewers: kuhar, brzycki, dmgreen

Reviewed By: kuhar

Author: NutshellySima

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D48923

llvm-svn: 336294