[VPlan] Simplify VPDerivedIV truncation handling (NFCI).

Address post-commit simplification suggestion for 8a56179: Replace
IsTruncated by conditionally setting TruncResultTy only if truncation
is required.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -9488,10 +9488,11 @@ void VPDerivedIVRecipe::execute(VPTransformState &State) {
                                           getStartValue()->getLiveInIRValue(),
                                           Step, Kind, BinOp);
   DerivedIV->setName("offset.idx");
-  if (ResultTy != DerivedIV->getType()) {
-    assert(IsTruncated && Step->getType()->isIntegerTy() &&
+  if (TruncResultTy) {
+    assert(TruncResultTy != DerivedIV->getType() &&
+           Step->getType()->isIntegerTy() &&
            "Truncation requires an integer step");
-    DerivedIV = State.Builder.CreateTrunc(DerivedIV, ResultTy);
+    DerivedIV = State.Builder.CreateTrunc(DerivedIV, TruncResultTy);
   }
   assert(DerivedIV != CanonicalIV && "IV didn't need transforming?");
 

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -2139,24 +2139,21 @@ class VPWidenCanonicalIVRecipe : public VPRecipeBase, public VPValue {
 /// an IV with different start and step values, using Start + CanonicalIV *
 /// Step.
 class VPDerivedIVRecipe : public VPRecipeBase, public VPValue {
-  /// The type of the result value. It may be smaller than the type of the
-  /// induction and in this case it will get truncated to ResultTy.
-  Type *ResultTy;
-
-  bool IsTruncated;
+  /// If not nullptr, the result of the induction will get truncated to
+  /// TruncResultTy.
+  Type *TruncResultTy;
 
-  /// Induction descriptor for the induction the canonical IV is transformed to.
+  /// Kind and binary operator of the induction.
   const InductionDescriptor::InductionKind Kind;
   const BinaryOperator *BinOp;
 
 public:
   VPDerivedIVRecipe(const InductionDescriptor &IndDesc, VPValue *Start,
                     VPCanonicalIVPHIRecipe *CanonicalIV, VPValue *Step,
-                    Type *ResultTy)
+                    Type *TruncResultTy)
       : VPRecipeBase(VPDef::VPDerivedIVSC, {Start, CanonicalIV, Step}),
-        VPValue(this), ResultTy(ResultTy),
-        IsTruncated(IndDesc.getStep()->getType() != ResultTy),
-        Kind(IndDesc.getKind()), BinOp(IndDesc.getInductionBinOp()) {}
+        VPValue(this), TruncResultTy(TruncResultTy), Kind(IndDesc.getKind()),
+        BinOp(IndDesc.getInductionBinOp()) {}
 
   ~VPDerivedIVRecipe() override = default;
 

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -797,8 +797,8 @@ void VPDerivedIVRecipe::print(raw_ostream &O, const Twine &Indent,
   O << " * ";
   getStepValue()->printAsOperand(O, SlotTracker);
 
-  if (IsTruncated)
-    O << " (truncated to " << *ResultTy << ")";
+  if (TruncResultTy)
+    O << " (truncated to " << *TruncResultTy << ")";
 }
 #endif
 

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -511,7 +511,8 @@ static VPValue *createScalarIVSteps(VPlan &Plan, const InductionDescriptor &ID,
   Type *TruncTy = TruncI ? TruncI->getType() : IVTy;
   VPValue *BaseIV = CanonicalIV;
   if (!CanonicalIV->isCanonical(ID.getKind(), StartV, Step, TruncTy)) {
-    BaseIV = new VPDerivedIVRecipe(ID, StartV, CanonicalIV, Step, TruncTy);
+    BaseIV = new VPDerivedIVRecipe(ID, StartV, CanonicalIV, Step,
+                                   TruncI ? TruncI->getType() : nullptr);
     HeaderVPBB->insert(BaseIV->getDefiningRecipe(), IP);
   }