Revert "[SimplifyCFG] Stop inserting calls to llvm.trap for UB"

This reverts commit r273778, it seems to break UBSan :/

llvm-svn: 273779
diff --git a/llvm/lib/Transforms/Utils/LoopSimplify.cpp b/llvm/lib/Transforms/Utils/LoopSimplify.cpp
index a2dd581..2509b0a 100644
--- a/llvm/lib/Transforms/Utils/LoopSimplify.cpp
+++ b/llvm/lib/Transforms/Utils/LoopSimplify.cpp
@@ -491,7 +491,7 @@
 
       // Zap the dead pred's terminator and replace it with unreachable.
       TerminatorInst *TI = P->getTerminator();
-      changeToUnreachable(TI);
+      changeToUnreachable(TI, /*UseLLVMTrap=*/false);
       Changed = true;
     }
   }