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

[courbet]

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"

[jdoerfert]

Do we have a test for this? I don't think it is a problem but if we do we could update it. Otherwise, LGTM.

[courbet]

Do we have a test for this? I don't think it is a problem but if we do we could update it. Otherwise, LGTM.

We don't. This is just for debug IIUC.

[courbet]

Thanks.

llvm/include/llvm/Support/GenericDomTree.h

void print(raw_ostream &O) const {
     else
       O << "Inorder Dominator Tree: ";
     if (!DFSInfoValid)
       O << "DFSNumbers invalid: " << SlowQueries << " slow queries.";
     O << "\n";
 
     // The postdom tree can have a null root if there are no returns.
     if (getRootNode()) PrintDomTree<NodeT>(getRootNode(), O, 1);
-    if (IsPostDominator) {
-      O << "Roots: ";
-      for (const NodePtr Block : Roots) {
-        Block->printAsOperand(O, false);
-        O << " ";
-      }
-      O << "\n";
-    }
-  }
 
 public:
   /// updateDFSNumbers - Assign In and Out numbers to the nodes while walking
   /// dominator tree in dfs order.
   void updateDFSNumbers() const {
     if (DFSInfoValid) {
       SlowQueries = 0;
       return;