[VPlan] Use VPDef for VPWidenGEPRecipe.

[fhahn]

Summary

This patch turns updates VPWidenGEPRecipe to manage the value it defines
using VPDef. The VPValue is used during VPlan construction and
codegeneration instead of the plain IR reference where possible.

[fhahn]

Updated to keep VPWidenGEPRecipe a VPValue which is then added to its VPDef. This simplifies things a bit.

[fhahn]

Update after VPDef does not inherit from VPUser directly.

[fhahn]

ping

[fhahn]

rebase before landing

llvm/lib/Transforms/Vectorize/VPlan.h

public:
   void execute(VPTransformState &State) override;
 
   /// Print the recipe.
   void print(raw_ostream &O, const Twine &Indent,
              VPSlotTracker &SlotTracker) const override;
 };
 
 /// A recipe for handling GEP instructions.
-class VPWidenGEPRecipe : public VPRecipeBase, public VPValue, public VPUser {
+class VPWidenGEPRecipe : public VPRecipeBase,
+                         public VPDef,
+                         public VPUser,
+                         public VPValue {
   bool IsPtrLoopInvariant;
   SmallBitVector IsIndexLoopInvariant;
 
 public:
   template <typename IterT>
   VPWidenGEPRecipe(GetElementPtrInst *GEP, iterator_range<IterT> Operands)
-      : VPRecipeBase(VPRecipeBase::VPWidenGEPSC), VPValue(VPWidenGEPSC, GEP),
-        VPUser(Operands), IsIndexLoopInvariant(GEP->getNumIndices(), false) {}
+      : VPRecipeBase(VPRecipeBase::VPWidenGEPSC), VPUser(Operands),
+        VPValue(VPWidenGEPSC, GEP, this),
+        IsIndexLoopInvariant(GEP->getNumIndices(), false) {}
 
   template <typename IterT>
   VPWidenGEPRecipe(GetElementPtrInst *GEP, iterator_range<IterT> Operands,
                    Loop *OrigLoop)
-      : VPRecipeBase(VPRecipeBase::VPWidenGEPSC),
-        VPValue(VPValue::VPVWidenGEPSC, GEP), VPUser(Operands),
+      : VPRecipeBase(VPRecipeBase::VPWidenGEPSC), VPUser(Operands),
+        VPValue(VPValue::VPVWidenGEPSC, GEP, this),
         IsIndexLoopInvariant(GEP->getNumIndices(), false) {
     IsPtrLoopInvariant = OrigLoop->isLoopInvariant(GEP->getPointerOperand());
     for (auto Index : enumerate(GEP->indices()))
       IsIndexLoopInvariant[Index.index()] =
           OrigLoop->isLoopInvariant(Index.value().get());
   }
   ~VPWidenGEPRecipe() override = default;