[NVPTX] Fix compiler warning in NDEBUG build [NFC]

Without the fix we get

../lib/Target/NVPTX/NVPTXLowerArgs.cpp:236:24: error: lambda capture 'Arg' is not used [-Werror,-Wunused-lambda-capture]
  auto IsALoadChain = [Arg](Value *Start) {
                       ^~~
1 error generated.