[AMDGPU] Fix -Wunused-variable in AMDGPUAtomicOptimizer.cpp (NFC)
/llvm-project/llvm/lib/Target/AMDGPU/AMDGPUAtomicOptimizer.cpp:688:18:
error: unused variable 'TyBitWidth' [-Werror,-Wunused-variable]
const unsigned TyBitWidth = DL->getTypeSizeInBits(Ty);
^
1 error generated.