[Dominators] Add isUpdateLazy() method to the DomTreeUpdater

Summary:
Previously, when people need to deal with DTU with different UpdateStrategy using different actions, they need to
```
if (DTU.getUpdateStrategy() == DomTreeUpdater::UpdateStrategy::Lazy) {
  ...
}
if (DTU.getUpdateStrategy() == DomTreeUpdater::UpdateStrategy::Eager) {
  ...
}
```
After the patch, they can avoid code patterns above
```
if (DTU.isUpdateLazy()){
  ...
}
if (!DTU.isUpdateLazy()){
  ...
}
```

Reviewers: kuhar, brzycki, dmgreen

Reviewed By: kuhar

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D49056

llvm-svn: 336886