[mlir][Vector] Fix -Wunused-function in VectorEmulateNarrowType.cpp (NFC)

/data/llvm-project/mlir/lib/Dialect/Vector/Transforms/VectorEmulateNarrowType.cpp:229:21: error: unused function 'operator<<' [-Werror,-Wunused-function]
static raw_ostream &operator<<(raw_ostream &os,
                    ^
1 error generated.

Author: DamonFool

mlir/lib/Dialect/Vector/Transforms/VectorEmulateNarrowType.cpp

@@ -226,7 +226,7 @@ struct BitCastBitsEnumerator {
 
 } // namespace
 
-static raw_ostream &operator<<(raw_ostream &os,
+[[maybe_unused]] static raw_ostream &operator<<(raw_ostream &os,
                                const SmallVector<SourceElementRangeList> &vec) {
   for (const auto &l : vec) {
     for (auto it : llvm::enumerate(l)) {