[ShapeDialect] Silence a build warning, NFC

mlir/lib/Dialect/Shape/IR/Shape.cpp:573:26: warning: loop variable 'shape' is always a copy because the range of type '::mlir::Operation::operand_range' (aka 'mlir::OperandRange') does not return a reference [-Wrange-loop-analysis]
        for (const auto &shape : shapes()) {
                         ^