[RISCV] Use std::optional in RISCVGatherScatterLowering.cpp (NFC)

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

llvm/lib/Target/RISCV/RISCVGatherScatterLowering.cpp

@@ -23,6 +23,7 @@
 #include "llvm/IR/IntrinsicsRISCV.h"
 #include "llvm/IR/PatternMatch.h"
 #include "llvm/Transforms/Utils/Local.h"
+#include <optional>
 
 using namespace llvm;
 using namespace PatternMatch;
@@ -346,7 +347,7 @@ RISCVGatherScatterLowering::determineBaseAndStride(GetElementPtrInst *GEP,
   if (Ops[0]->getType()->isVectorTy())
     return std::make_pair(nullptr, nullptr);
 
-  Optional<unsigned> VecOperand;
+  std::optional<unsigned> VecOperand;
   unsigned TypeScale = 0;
 
   // Look for a vector operand and scale.