[RISCV] Fix -Wdeprecated-declarations in RISCVInstrInfo.cpp (NFC)

/Users/jiefu/llvm-project/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp:2636:10: error: 'makeArrayRef<std::pair<llvm::MachineMemOperand::Flags, const char *>, 2UL>' is deprecated: Use deduction guide instead [-Werror,-Wdeprecated-declarations]
  return makeArrayRef(TargetFlags);
         ^~~~~~~~~~~~
         ArrayRef
/Users/jiefu/llvm-project/llvm/include/llvm/ADT/ArrayRef.h:572:3: note: 'makeArrayRef<std::pair<llvm::MachineMemOperand::Flags, const char *>, 2UL>' has been explicitly marked deprecated here
  LLVM_DEPRECATED("Use deduction guide instead", "ArrayRef")
  ^
/Users/jiefu/llvm-project/llvm/include/llvm/Support/Compiler.h:143:50: note: expanded from macro 'LLVM_DEPRECATED'
                                                 ^
1 error generated.