[mlir] Format AffineOps.td. NFC

       }

    ```

    "loop.for" can also operate on loop-carried variables and returns the final values

    after loop termination. The initial values of the variables are passed as additional SSA

    operands to the "loop.for" following the 3 loop control SSA values mentioned above

    (lower bound, upper bound and step). The operation region has equivalent arguments

    for each variable representing the value of the variable at the current iteration.

    The region must terminate with a "loop.yield" that passes all the current iteration

    variables to the next iteration, or to the "loop.for" result, if at the last iteration.

    "loop.for" results hold the final values after the last iteration.

    "loop.for" can also operate on loop-carried variables and returns the final

    values after loop termination. The initial values of the variables are

    passed as additional SSA operands to the "loop.for" following the 3 loop

    control SSA values mentioned above (lower bound, upper bound and step). The

    operation region has equivalent arguments for each variable representing

    the value of the variable at the current iteration.

    The region must terminate with a "loop.yield" that passes all the current

    iteration variables to the next iteration, or to the "loop.for" result, if

    at the last iteration.  "loop.for" results hold the final values after the

    last iteration.

    For example, to sum-reduce a memref:

    ```mlir

    func @reduce(%buffer: memref<1024xf32>, %lb: index, %ub: index, %step: index) -> (f32) {

    func @reduce(%buffer: memref<1024xf32>, %lb: index,

                 %ub: index, %step: index) -> (f32) {

      // Initial sum set to 0.

      %sum_0 = constant 0.0 : f32

      // iter_args binds initial values to the loop's region arguments.

      %sum = loop.for %iv = %lb to %ub step %step iter_args(%sum_iter = %sum_0) -> (f32) {

      %sum = loop.for %iv = %lb to %ub step %step

          iter_args(%sum_iter = %sum_0) -> (f32) {

        %t = load %buffer[%iv] : memref<1024xf32>

        %sum_next = addf %sum_iter, %t : f32

        // Yield current iteration sum to next iteration %sum_iter or to %sum if final iteration.

        // Yield current iteration sum to next iteration %sum_iter or to %sum

        // if final iteration.

        loop.yield %sum_next : f32

      }

      return %sum : f32

    ```

    If the "loop.for" defines any values, a yield must be explicitly present.

    The number and types of the "loop.for" results must match the initial values

    in the "iter_args" binding and the yield operands.

    The number and types of the "loop.for" results must match the initial

    values in the "iter_args" binding and the yield operands.

    Another example with a nested "loop.if" (see "loop.if" for details)

    to perform conditional reduction:

    ```mlir

    func @conditional_reduce(%buffer: memref<1024xf32>, %lb: index, %ub: index, %step: index) -> (f32) {

    func @conditional_reduce(%buffer: memref<1024xf32>, %lb: index,

                             %ub: index, %step: index) -> (f32) {

      %sum_0 = constant 0.0 : f32

      %c0 = constant 0.0 : f32

      %sum = loop.for %iv = %lb to %ub step %step iter_args(%sum_iter = %sum_0) -> (f32) {

      %sum = loop.for %iv = %lb to %ub step %step

          iter_args(%sum_iter = %sum_0) -> (f32) {

        %t = load %buffer[%iv] : memref<1024xf32>

        %cond = cmpf "ugt", %t, %c0 : f32

        %sum_next = loop.if %cond -> (f32) {

       }

    ```

    "loop.if" may also return results that are defined in its regions. The values

    defined are determined by which execution path is taken.

    "loop.if" may also return results that are defined in its regions. The

    values defined are determined by which execution path is taken.

    For example:

    ```mlir

       %x, %y = loop.if %b -> (f32, f32) {

    The body region must contain exactly one block that terminates with

    "loop.yield" without operands. Parsing ParallelOp will create such a region

    and insert the terminator when it is absent from the custom format. For example:

    and insert the terminator when it is absent from the custom format.

    For example:

    ```mlir

       loop.parallel (%iv) = (%lb) to (%ub) step (%step) {

  let summary = "terminator for reduce operation";

  let description = [{

    "loop.reduce.return" is a special terminator operation for the block inside

    "loop.reduce". It terminates the region. It should have the same type as the

    operand of "loop.reduce". Example for the custom format:

    "loop.reduce". It terminates the region. It should have the same type as

    the operand of "loop.reduce". Example for the custom format:

    ```mlir

      loop.reduce.return %res : f32

  let arguments = (ins Variadic<AnyType>:$results);

  let builders = [

    OpBuilder<"Builder *builder, OperationState &result", [{ /* nothing to do */ }]>

    OpBuilder<"Builder *builder, OperationState &result",

              [{ /* nothing to do */ }]>

  ];

}

#endif // LOOP_OPS