Update after ArrayRef(std::nullopt) deprecation (#3248)

Update for llvm-project commit 2529de5 ("[ADT] Deprecate
ArrayRef(std::nullopt) (#146011)", 2025-06-27).

Original commit:
KhronosGroup/SPIRV-LLVM-Translator@e96d63a89c31ed4

Author: svenvh

llvm-spirv/lib/SPIRV/SPIRVReader.cpp

@@ -702,7 +702,7 @@ void SPIRVToLLVM::setLLVMLoopMetadata(const LoopInstType *LM,
   if (!LM)
     return;
 
-  auto Temp = MDNode::getTemporary(*Context, std::nullopt);
+  auto Temp = MDNode::getTemporary(*Context, {});
   auto *Self = MDNode::get(*Context, Temp.get());
   Self->replaceOperandWith(0, Self);
   SPIRVWord LC = LM->getLoopControl();
@@ -852,8 +852,7 @@ void SPIRVToLLVM::setLLVMLoopMetadata(const LoopInstType *LM,
           // Emit a distinct index group that will be referenced from
           // llvm.loop.parallel_access_indices metadata; hash the new
           // MDNode for future accesses to the same memory.
-          CurrentDepthIdxGroup =
-              llvm::MDNode::getDistinct(*Context, std::nullopt);
+          CurrentDepthIdxGroup = llvm::MDNode::getDistinct(*Context, {});
           OffsetIdxGroupMap.emplace(Info, CurrentDepthIdxGroup);
         } else {
           // Previous accesses to that field have already been indexed,
@@ -863,8 +862,7 @@ void SPIRVToLLVM::setLLVMLoopMetadata(const LoopInstType *LM,
       } else /* Regular kernel-scope array/pointer variable */ {
         // Emit a distinct index group that will be referenced from
         // llvm.loop.parallel_access_indices metadata
-        CurrentDepthIdxGroup =
-            llvm::MDNode::getDistinct(*Context, std::nullopt);
+        CurrentDepthIdxGroup = llvm::MDNode::getDistinct(*Context, {});
       }
 
       unsigned Safelen = PointerSflnMap.find(ArrayGEPIt.first)->second;