[IVDesc] Make getCastInsts return an ArrayRef (NFC) (#169021)

To make it clear that the return value is immutable.

Author: artagnon

llvm/include/llvm/Analysis/IVDescriptors.h

@@ -451,12 +451,10 @@ class InductionDescriptor {
                           : Instruction::BinaryOpsEnd;
   }
 
-  /// Returns a reference to the type cast instructions in the induction
+  /// Returns an ArrayRef to the type cast instructions in the induction
   /// update chain, that are redundant when guarded with a runtime
   /// SCEV overflow check.
-  const SmallVectorImpl<Instruction *> &getCastInsts() const {
-    return RedundantCasts;
-  }
+  ArrayRef<Instruction *> getCastInsts() const { return RedundantCasts; }
 
 private:
   /// Private constructor - used by \c isInductionPHI.

llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp

@@ -684,7 +684,7 @@ void LoopVectorizationLegality::addInductionPhi(
   // in the vectorized loop body, record them here. All casts could be recorded
   // here for ignoring, but suffices to record only the first (as it is the
   // only one that may bw used outside the cast sequence).
-  const SmallVectorImpl<Instruction *> &Casts = ID.getCastInsts();
+  ArrayRef<Instruction *> Casts = ID.getCastInsts();
   if (!Casts.empty())
     InductionCastsToIgnore.insert(*Casts.begin());
 

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -6575,8 +6575,7 @@ void LoopVectorizationCostModel::collectValuesToIgnore() {
   // detection.
   for (const auto &Induction : Legal->getInductionVars()) {
     const InductionDescriptor &IndDes = Induction.second;
-    const SmallVectorImpl<Instruction *> &Casts = IndDes.getCastInsts();
-    VecValuesToIgnore.insert_range(Casts);
+    VecValuesToIgnore.insert_range(IndDes.getCastInsts());
   }
 }
 

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -512,7 +512,7 @@ static void removeRedundantInductionCasts(VPlan &Plan) {
     // replace it with the original IV. Note that only the final cast is
     // expected to have users outside the cast-chain and the dead casts left
     // over will be cleaned up later.
-    auto &Casts = IV->getInductionDescriptor().getCastInsts();
+    ArrayRef<Instruction *> Casts = IV->getInductionDescriptor().getCastInsts();
     VPValue *FindMyCast = IV;
     for (Instruction *IRCast : reverse(Casts)) {
       VPSingleDefRecipe *FoundUserCast = nullptr;