[mlir] Fix -Wunused-variable in XeGPUOps.cpp (NFC)

llvm-project/mlir/lib/Dialect/XeGPU/IR/XeGPUOps.cpp:47:8:
error: unused variable 'ty' [-Werror,-Wunused-variable]
  auto ty = source.getType();
       ^
1 error generated.