[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.

Author: DamonFool

mlir/lib/Dialect/XeGPU/IR/XeGPUOps.cpp

@@ -44,7 +44,7 @@ static std::string makeString(T array, bool breakline = false) {
 void CreateNdDescOp::build(OpBuilder &builder, OperationState &state,
                            Type tdesc, TypedValue<MemRefType> source,
                            llvm::ArrayRef<OpFoldResult> offsets) {
-  auto ty = source.getType();
+  [[maybe_unused]] auto ty = source.getType();
   assert(ty.hasStaticShape() && offsets.size() == (size_t)ty.getRank());
 
   llvm::SmallVector<int64_t> staticOffsets;