[Dominators] Take fast path when applying <=1 updates

Summary:
This patch teaches `DT.applyUpdates` to take the fast when applying zero or just one update and makes it not run the internal batch updater machinery.

With this patch, it should no longer make sense to have a special check in user's code that checks the update sequence size before applying them, e.g.
```
if (!MyUpdates.empty())
  DT.applyUpdates(MyUpdates);
```
or
```
if (MyUpdates.size() == 1)
  if (...)
    DT.insertEdge(...)
  else
    DT.deleteEdge(...)
```

Reviewers: dberlin, brzycki, davide, grosser, sanjoy

Reviewed By: dberlin, davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38541

llvm-svn: 314917