Repair compilation of llvm-stress after r350835.

Apparently it doesn't get built by 'ninja check'. :(

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350843 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/tools/llvm-stress/llvm-stress.cpp b/tools/llvm-stress/llvm-stress.cpp
index f7aff19..c29b7a7 100644
--- a/tools/llvm-stress/llvm-stress.cpp
+++ b/tools/llvm-stress/llvm-stress.cpp
@@ -356,7 +356,7 @@
   void Act() override {
     // Try to use predefined pointers. If non-exist, use undef pointer value;
     Value *Ptr = getRandomPointerValue();
-    PointerType *Tp = Ptr->getType();
+    PointerType *Tp = cast<PointerType>(Ptr->getType());
     Value *Val = getRandomValue(Tp->getElementType());
     Type  *ValTy = Val->getType();