[VPlan] Clarify setting Lane in fixPhi (NFCI).

Split off from #94760, clarify
as suggested.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -189,10 +189,10 @@ bool VPRecipeBase::mayHaveSideEffects() const {
 }
 
 void VPLiveOut::fixPhi(VPlan &Plan, VPTransformState &State) {
-  auto Lane = VPLane::getLastLaneForVF(State.VF);
   VPValue *ExitValue = getOperand(0);
-  if (vputils::isUniformAfterVectorization(ExitValue))
-    Lane = VPLane::getFirstLane();
+  auto Lane = vputils::isUniformAfterVectorization(ExitValue)
+                  ? VPLane::getFirstLane()
+                  : VPLane::getLastLaneForVF(State.VF);
   VPBasicBlock *MiddleVPBB =
       cast<VPBasicBlock>(Plan.getVectorLoopRegion()->getSingleSuccessor());
   BasicBlock *MiddleBB = State.CFG.VPBB2IRBB[MiddleVPBB];