[VPlan] Add new VPPhi subclass for VPInstruction with PHI opcodes.

Similarly to VPInstructionWithType and VPIRPhi, add VPPhi as a subclass
for VPInstruction. This allows implementing the VPPhiAccessors trait,
making available helpers for generic printing of incoming values /
blocks and accessors for incoming blocks and values.

It will also allow properly verifying def-uses for values used by
VPInstructions with PHI opcodes via
#124838.

llvm/lib/Transforms/Vectorize/LoopVectorizationPlanner.h

@@ -253,7 +253,7 @@ class VPBuilder {
   VPInstruction *createScalarPhi(ArrayRef<VPValue *> IncomingValues,
                                  DebugLoc DL, const Twine &Name = "") {
     return tryInsertInstruction(
-        new VPInstruction(Instruction::PHI, IncomingValues, DL, Name));
+        new VPPhi(Instruction::PHI, IncomingValues, DL, Name));
   }
 
   /// Convert the input value \p Current to the corresponding value of an

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -1143,6 +1143,28 @@ class VPPhiAccessors {
 #endif
 };
 
+struct VPPhi : public VPInstruction, public VPPhiAccessors {
+  VPPhi(unsigned Opcode, ArrayRef<VPValue *> Operands, DebugLoc DL,
+        const Twine &Name = "")
+      : VPInstruction(Opcode, Operands, DL, Name) {}
+
+  static inline bool classof(const VPRecipeBase *U) {
+    auto *R = dyn_cast<VPInstruction>(U);
+    return R && R->getOpcode() == Instruction::PHI;
+  }
+
+  void execute(VPTransformState &State) override;
+
+#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
+  /// Print the recipe.
+  void print(raw_ostream &O, const Twine &Indent,
+             VPSlotTracker &SlotTracker) const override;
+#endif
+
+protected:
+  const VPRecipeBase *getAsRecipe() const override { return this; }
+};
+
 /// A recipe to wrap on original IR instruction not to be modified during
 /// execution, except for PHIs. PHIs are modeled via the VPIRPhi subclass.
 /// Expect PHIs, VPIRInstructions cannot have any operands.

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -492,16 +492,7 @@ Value *VPInstruction::generate(VPTransformState &State) {
     return Builder.CreateCmp(getPredicate(), A, B, Name);
   }
   case Instruction::PHI: {
-    assert(getParent() ==
-               getParent()->getPlan()->getVectorLoopRegion()->getEntry() &&
-           "VPInstructions with PHI opcodes must be used for header phis only "
-           "at the moment");
-    BasicBlock *VectorPH =
-        State.CFG.VPBB2IRBB.at(getParent()->getCFGPredecessor(0));
-    Value *Start = State.get(getOperand(0), VPLane(0));
-    PHINode *Phi = State.Builder.CreatePHI(Start->getType(), 2, Name);
-    Phi->addIncoming(Start, VectorPH);
-    return Phi;
+    llvm_unreachable("should be handled by VPPhi::execute");
   }
   case Instruction::Select: {
     bool OnlyFirstLaneUsed = vputils::onlyFirstLaneUsed(this);
@@ -1131,6 +1122,30 @@ void VPInstructionWithType::print(raw_ostream &O, const Twine &Indent,
 }
 #endif
 
+void VPPhi::execute(VPTransformState &State) {
+  State.setDebugLocFrom(getDebugLoc());
+  assert(getParent() ==
+             getParent()->getPlan()->getVectorLoopRegion()->getEntry() &&
+         "VPInstructions with PHI opcodes must be used for header phis only "
+         "at the moment");
+  BasicBlock *VectorPH = State.CFG.VPBB2IRBB.at(getIncomingBlock(0));
+  Value *Start = State.get(getIncomingValue(0), VPLane(0));
+  PHINode *Phi = State.Builder.CreatePHI(Start->getType(), 2, getName());
+  Phi->addIncoming(Start, VectorPH);
+  State.set(this, Phi, VPLane(0));
+}
+
+#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
+void VPPhi::print(raw_ostream &O, const Twine &Indent,
+                  VPSlotTracker &SlotTracker) const {
+  O << Indent << "EMIT ";
+  printAsOperand(O, SlotTracker);
+  O << " = phi ";
+
+  printPhiOperands(O, SlotTracker);
+}
+#endif
+
 VPIRInstruction *VPIRInstruction ::create(Instruction &I) {
   if (auto *Phi = dyn_cast<PHINode>(&I))
     return new VPIRPhi(*Phi);