[mlir] Fix -Wunused-result in ElementwiseOpFusion.cpp (NFC)
/llvm-project/mlir/lib/Dialect/Linalg/Transforms/ElementwiseOpFusion.cpp:124:7:
error: ignoring return value of function declared with 'nodiscard' attribute [-Werror,-Wunused-result]
opOperandsToIgnore.pop_back_val();
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.