[DominatorTree] Print roots unconditionally in `print()`.

Summary:
This came up in a debugging session. I was failing to update the root of
the tree, and got during verification:

```
DominatorTree is different than a freshly computed one!
        Current:
=============================--------------------------------
Inorder Dominator Tree: DFSNumbers invalid: 0 slow queries.
  [1] %"entry+land.rhs.i" {4294967295,4294967295} [0]
    [2] %opeq1.exit {4294967295,4294967295} [1]

        Freshly computed tree:
=============================--------------------------------
Inorder Dominator Tree: DFSNumbers invalid: 0 slow queries.
  [1] %"entry+land.rhs.i" {4294967295,4294967295} [0]
    [2] %opeq1.exit {4294967295,4294967295} [1]
```

We now print:

```
DominatorTree is different than a freshly computed one!
        Current:
=============================--------------------------------
Inorder Dominator Tree: DFSNumbers invalid: 0 slow queries.
  [1] %"entry+land.rhs.i" {4294967295,4294967295} [0]
    [2] %opeq1.exit {4294967295,4294967295} [1]
Roots: <badref>

        Freshly computed tree:
=============================--------------------------------
Inorder Dominator Tree: DFSNumbers invalid: 0 slow queries.
  [1] %"entry+land.rhs.i" {4294967295,4294967295} [0]
    [2] %opeq1.exit {4294967295,4294967295} [1]
Roots: %"entry+land.rhs.i"
```

Reviewers: kuhar, asbirlea

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61999

llvm-svn: 360886