[VPlan] VPEVLBasedIVPHI is a VPSingleDefRecipe.

VPEVLBasedIVPHIRecipe inherits from VPSingleDefRecipe. Add
VPEVLBasedIVPHISC to VPSingleDefRecipe::classof to make isa/dyn_cast &
co work as expected.

Split off #67934.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -841,6 +841,7 @@ class VPSingleDefRecipe : public VPRecipeBase, public VPValue {
   static inline bool classof(const VPRecipeBase *R) {
     switch (R->getVPDefID()) {
     case VPRecipeBase::VPDerivedIVSC:
+    case VPRecipeBase::VPEVLBasedIVPHISC:
     case VPRecipeBase::VPExpandSCEVSC:
     case VPRecipeBase::VPInstructionSC:
     case VPRecipeBase::VPReductionSC:

llvm/unittests/Transforms/Vectorize/VPlanTest.cpp

@@ -1530,5 +1530,13 @@ TEST(VPDoubleValueDefTest, traverseUseLists) {
   EXPECT_EQ(&DoubleValueDef, I3.getOperand(0)->getDefiningRecipe());
 }
 
+TEST(VPRecipeTest, CastToVPSingleDefRecipe) {
+  VPValue Start;
+  VPEVLBasedIVPHIRecipe R(&Start, {});
+  VPRecipeBase *B = &R;
+  EXPECT_TRUE(isa<VPSingleDefRecipe>(B));
+  // TODO: check other VPSingleDefRecipes.
+}
+
 } // namespace
 } // namespace llvm