[LV] Retain branch in middle block when scalar epilogue is needed (NFC)

splitBlock will create a unconditional branch between the middle block
and scalar preheader. Instead of creating and replacing the same branch
again when scalar epilogue is needed, simply add an early exit.

As suggested by @ayalz in
#92651 to clarify the existing
code.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -2971,32 +2971,33 @@ void InnerLoopVectorizer::createVectorLoopSkeleton(StringRef Prefix) {
       SplitBlock(LoopMiddleBlock, LoopMiddleBlock->getTerminator(), DT, LI,
                  nullptr, Twine(Prefix) + "scalar.ph");
 
-  auto *ScalarLatchTerm = OrigLoop->getLoopLatch()->getTerminator();
-
   // Set up the middle block terminator.  Two cases:
-  // 1) If we know that we must execute the scalar epilogue, emit an
-  //    unconditional branch.
+  // 1) If we know that we must execute the scalar epilogue, retain the existing
+  // unconditional branch from the middle block to the scalar preheader. In that
+  // case, there's no edge from the middle block to exit blocks  and thus no
+  // need to update the immediate dominator of the exit blocks.
+  if (Cost->requiresScalarEpilogue(VF.isVector())) {
+    assert(
+        LoopMiddleBlock->getSingleSuccessor() == LoopScalarPreHeader &&
+        " middle block should have the scalar preheader as single successor");
+    return;
+  }
+
   // 2) Otherwise, we must have a single unique exit block (due to how we
   //    implement the multiple exit case).  In this case, set up a conditional
   //    branch from the middle block to the loop scalar preheader, and the
   //    exit block.  completeLoopSkeleton will update the condition to use an
   //    iteration check, if required to decide whether to execute the remainder.
   BranchInst *BrInst =
-      Cost->requiresScalarEpilogue(VF.isVector())
-          ? BranchInst::Create(LoopScalarPreHeader)
-          : BranchInst::Create(LoopExitBlock, LoopScalarPreHeader,
-                               Builder.getTrue());
+      BranchInst::Create(LoopExitBlock, LoopScalarPreHeader, Builder.getTrue());
+  auto *ScalarLatchTerm = OrigLoop->getLoopLatch()->getTerminator();
   BrInst->setDebugLoc(ScalarLatchTerm->getDebugLoc());
   ReplaceInstWithInst(LoopMiddleBlock->getTerminator(), BrInst);
 
   // Update dominator for loop exit. During skeleton creation, only the vector
   // pre-header and the middle block are created. The vector loop is entirely
   // created during VPlan exection.
-  if (!Cost->requiresScalarEpilogue(VF.isVector()))
-    // If there is an epilogue which must run, there's no edge from the
-    // middle block to exit blocks  and thus no need to update the immediate
-    // dominator of the exit blocks.
-    DT->changeImmediateDominator(LoopExitBlock, LoopMiddleBlock);
+  DT->changeImmediateDominator(LoopExitBlock, LoopMiddleBlock);
 }
 
 PHINode *InnerLoopVectorizer::createInductionResumeValue(