[runtimeunroll] Fix reported DT verification error after 94d0914

In 94d0914, I added support for unrolling of multiple exit loops which have multiple exits reaching the latch.  Per reports on the review post commit, I'd missed updating the domtree for one case.  This fix addresses that ommission.

There's no new test as this is covered by existing tests with expensive verification turned on.

llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp

@@ -285,8 +285,10 @@ static void ConnectEpilog(Loop *L, Value *ModVal, BasicBlock *NewExit,
   // Add the branch to the exit block (around the unrolling loop)
   B.CreateCondBr(BrLoopExit, EpilogPreHeader, Exit);
   InsertPt->eraseFromParent();
-  if (DT)
-    DT->changeImmediateDominator(Exit, NewExit);
+  if (DT) {
+    auto *NewDom = DT->findNearestCommonDominator(Exit, NewExit);
+    DT->changeImmediateDominator(Exit, NewDom);
+  }
 
   // Split the main loop exit to maintain canonicalization guarantees.
   SmallVector<BasicBlock*, 4> NewExitPreds{Latch};