[VPlan] Use VPBasicBlock to get incoming block for exit phi fixup (NFC)

Retrieve block via VPlan infrastructure as suggested as independent
cleanup in D150398.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -156,8 +156,13 @@ void VPLiveOut::fixPhi(VPlan &Plan, VPTransformState &State) {
   VPValue *ExitValue = getOperand(0);
   if (vputils::isUniformAfterVectorization(ExitValue))
     Lane = VPLane::getFirstLane();
+  VPBasicBlock *MiddleVPBB =
+      cast<VPBasicBlock>(Plan.getVectorLoopRegion()->getSingleSuccessor());
+  assert(MiddleVPBB->getNumSuccessors() == 0 &&
+         "the middle block must not have any successors");
+  BasicBlock *MiddleBB = State.CFG.VPBB2IRBB[MiddleVPBB];
   Phi->addIncoming(State.get(ExitValue, VPIteration(State.UF - 1, Lane)),
-                   State.Builder.GetInsertBlock());
+                   MiddleBB);
 }
 
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)