[LoopUnroll] Don't update DT for changeToUnreachable.

[fhahn]

Summary

There is no need to update the DT here, because there must be a unique
latch. Hence if the latch is not exiting it must directly branch back
to the original loop header and does not dominate any nodes.

Skipping a DT update here simplifies D141487.

[nikic]

LGTM

llvm/lib/Transforms/Utils/LoopUnroll.cpp

for (unsigned i = 0, e = Info.ExitingBlocks.size(); i != e; ++i) {
       if (*KnownWillExit && !IsLatch)
         continue;
 
       SetDest(Info.ExitingBlocks[i], *KnownWillExit, Info.ExitOnTrue);
     }
   }
 
   // When completely unrolling, the last latch becomes unreachable.
-  if (!LatchIsExiting && CompletelyUnroll)
-    changeToUnreachable(Latches.back()->getTerminator(), PreserveLCSSA, &DTU);
+  if (!LatchIsExiting && CompletelyUnroll) {
+    // There is no need to update the DT here, because there must be a unique
+    // latch. Hence if the latch is not exiting it must directly branch back to
+    // the original loop header and does not dominate any nodes.
+    assert(LatchBlock->getSingleSuccessor() && "Loop with multiple latches?");
+    changeToUnreachable(Latches.back()->getTerminator(), PreserveLCSSA);
+  }
 
   // Merge adjacent basic blocks, if possible.
   for (BasicBlock *Latch : Latches) {
     BranchInst *Term = dyn_cast<BranchInst>(Latch->getTerminator());
     assert((Term ||
             (CompletelyUnroll && !LatchIsExiting && Latch == Latches.back())) &&
            "Need a branch as terminator, except when fully unrolling with "
            "unconditional latch");