[llvm] [VPlan] Add initial VPScalarEvolution, use to get trip count SCEV (NFC) (PR #94464)

[via llvm-commits]

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@@ -674,10 +674,8 @@ void VPlanTransforms::optimizeForVFAndUF(VPlan &Plan, ElementCount BestVF,
              m_BranchOnCond(m_Not(m_ActiveLaneMask(m_VPValue(), m_VPValue())))))
     return;
 
-  Type *IdxTy =
-      Plan.getCanonicalIV()->getStartValue()->getLiveInIRValue()->getType();
-  const SCEV *TripCount = createTripCountSCEV(IdxTy, PSE);
   ScalarEvolution &SE = *PSE.getSE();
+  const SCEV *TripCount = VPScalarEvolution(SE).getSCEV(Plan.getTripCount());
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ayalz wrote:

Suffice to have, in this case, as an alternative,
```suggestion
  VPValue *TripCountVPV = Plan.getTripCount();
  auto *Expand = dyn_cast<VPExpandSCEVRecipe>(TripCountVPV);
  const SCEV *TripCount = Expand ? Expand->getSCEV() : SE.getSCEV(TripCountVPV->getLiveInIRValue());
```
asserting TripCountVPV is either an Expand or a live-in?
Perhaps could be folded into Plan.getTripCountSCEV(SE), or have vputils::getSCEVExprForVPValue() complement vputils::getOrCreateVPValueForSCEVExpr().

https://github.com/llvm/llvm-project/pull/94464