[llvm] Migrate away from ArrayRef(std::nullopt) (NFC)

ArrayRef has a constructor that accepts std::nullopt.  This
constructor dates back to the days when we still had llvm::Optional.

Since the use of std::nullopt outside the context of std::optional is
kind of abuse and not intuitive to new comers, I would like to move
away from the constructor and eventually remove it.

This patch takes care of the llvm side of the migration.

Author: kazutakahirata

llvm/include/llvm/CodeGen/BasicTTIImpl.h

@@ -2376,8 +2376,8 @@ class BasicTTIImplBase : public TargetTransformInfoImplCRTPBase<T> {
                                           CostKind, 1, nullptr, nullptr);
       Cost += thisT()->getVectorInstrCost(Instruction::InsertElement, SearchTy,
                                           CostKind, 0, nullptr, nullptr);
-      Cost += thisT()->getShuffleCost(TTI::SK_Broadcast, SearchTy, std::nullopt,
-                                      CostKind, 0, nullptr);
+      Cost += thisT()->getShuffleCost(TTI::SK_Broadcast, SearchTy, {}, CostKind,
+                                      0, nullptr);
       Cost += thisT()->getCmpSelInstrCost(BinaryOperator::ICmp, SearchTy, RetTy,
                                           CmpInst::ICMP_EQ, CostKind);
       Cost +=

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -5994,7 +5994,7 @@ static bool isMaskedLoadCompress(
       InstructionCost InterleavedCost =
           VectorGEPCost + TTI.getInterleavedMemoryOpCost(
                               Instruction::Load, AlignedLoadVecTy,
-                              CompressMask[1], std::nullopt, CommonAlignment,
+                              CompressMask[1], {}, CommonAlignment,
                               LI->getPointerAddressSpace(), CostKind, IsMasked);
       if (InterleavedCost < GatherCost) {
         InterleaveFactor = CompressMask[1];
@@ -13561,7 +13561,7 @@ BoUpSLP::getEntryCost(const TreeEntry *E, ArrayRef<Value *> VectorizedVals,
       case TreeEntry::Vectorize:
         if (unsigned Factor = E->getInterleaveFactor()) {
           VecLdCost = TTI->getInterleavedMemoryOpCost(
-              Instruction::Load, VecTy, Factor, std::nullopt, LI0->getAlign(),
+              Instruction::Load, VecTy, Factor, {}, LI0->getAlign(),
               LI0->getPointerAddressSpace(), CostKind);
 
         } else {
@@ -13602,7 +13602,7 @@ BoUpSLP::getEntryCost(const TreeEntry *E, ArrayRef<Value *> VectorizedVals,
         Align CommonAlignment = LI0->getAlign();
         if (InterleaveFactor) {
           VecLdCost = TTI->getInterleavedMemoryOpCost(
-              Instruction::Load, LoadVecTy, InterleaveFactor, std::nullopt,
+              Instruction::Load, LoadVecTy, InterleaveFactor, {},
               CommonAlignment, LI0->getPointerAddressSpace(), CostKind);
         } else if (IsMasked) {
           VecLdCost = TTI->getMaskedMemoryOpCost(
@@ -13677,8 +13677,8 @@ BoUpSLP::getEntryCost(const TreeEntry *E, ArrayRef<Value *> VectorizedVals,
                  "No reused shuffles expected");
           CommonCost = 0;
           VecStCost = TTI->getInterleavedMemoryOpCost(
-              Instruction::Store, VecTy, Factor, std::nullopt,
-              BaseSI->getAlign(), BaseSI->getPointerAddressSpace(), CostKind);
+              Instruction::Store, VecTy, Factor, {}, BaseSI->getAlign(),
+              BaseSI->getPointerAddressSpace(), CostKind);
         } else {
           TTI::OperandValueInfo OpInfo = getOperandInfo(E->getOperand(0));
           VecStCost = TTI->getMemoryOpCost(

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -3478,8 +3478,7 @@ InstructionCost VPInterleaveRecipe::computeCost(ElementCount VF,
 
   return Cost + IG->getNumMembers() *
                     Ctx.TTI.getShuffleCost(TargetTransformInfo::SK_Reverse,
-                                           VectorTy, std::nullopt, Ctx.CostKind,
-                                           0);
+                                           VectorTy, {}, Ctx.CostKind, 0);
 }
 
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)