Fix dominator descendants for unreachable blocks.

When a block is unreachable, asking its dom tree descendants should
return the empty set. However, the computation of the descendants
was causing a segmentation fault because the dom tree node we get
from the basic block is initially NULL.

Fixed by adding a test for a valid dom tree node before we iterate.

The patch also adds some unit tests to the existing dom tree tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196099 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Analysis/Dominators.h

@@ -348,10 +348,12 @@ public:
 
   /// Get all nodes dominated by R, including R itself.
   void getDescendants(NodeT *R, SmallVectorImpl<NodeT *> &Result) const {
+    Result.clear();
     const DomTreeNodeBase<NodeT> *RN = getNode(R);
+    if (RN == NULL)
+      return; // If R is unreachable, it will not be present in the DOM tree.
     SmallVector<const DomTreeNodeBase<NodeT> *, 8> WL;
     WL.push_back(RN);
-    Result.clear();
 
     while (!WL.empty()) {
       const DomTreeNodeBase<NodeT> *N = WL.pop_back_val();