[NVPTX] Fix unused var warning with asserts disabled

<...>/llvm-project/llvm/lib/Target/NVPTX/NVPTXLowerArgs.cpp:191:15:
warning: unused variable ‘ASC’ [-Wunused-variable]
  191 |     if (auto *ASC =
dyn_cast<AddrSpaceCastInst>(I.OldInstruction)) {
      |               ^~~