[VPlan] Populate ExitBlocks when cloning VPlan (NFC).

Update VPlan::duplicate to add cloned exit blocks to ExitBlocks.

Currently there are no uses of the exit blocks after cloning so this is
NFC at the moment.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -1216,6 +1216,13 @@ VPlan *VPlan::duplicate() {
     NewPlan->CreatedBlocks.push_back(this->CreatedBlocks[I]);
   CreatedBlocks.truncate(NumBlocksBeforeCloning);
 
+  // Update ExitBlocks of the new plan.
+  for (VPBlockBase *VPB : NewPlan->CreatedBlocks) {
+    if (VPB->getNumSuccessors() == 0 && isa<VPIRBasicBlock>(VPB) &&
+        VPB != NewScalarHeader)
+      NewPlan->ExitBlocks.push_back(cast<VPIRBasicBlock>(VPB));
+  }
+
   return NewPlan;
 }