Add a linalg.for operation to support non-affine loop constructs

Add a linalg.for operation to support non-affine loop constructs

    The affine.for operation has restrictions that make it suitable for dependence analysis. The Linalg dialect aims at being more general.
    This CL introduces linalg.for, and its associated terminator, along with a simple roundtripping test.
    A `linalg.for` only takes one value of index type for lower bound, upper bound and step.

    Example usage:
    ```
    linalg.for %iv = %lb to %ub step %step {
      ... // body
    }
    ```

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PiperOrigin-RevId: 250369722