[LoopUnroll] Check that DT is available before trying to verify it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@272221 91177308-0d34-0410-b5e6-96231b3b80d8

Author: Michael Zolotukhin

lib/Transforms/Utils/LoopUnroll.cpp

@@ -602,7 +602,7 @@ bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount, bool Force,
   // updating domtree after partial loop unrolling should also be easy.
   if (DT && !CompletelyUnroll)
     DT->recalculate(*L->getHeader()->getParent());
-  else
+  else if (DT)
     DEBUG(DT->verifyDomTree());
 
   // Simplify any new induction variables in the partially unrolled loop.