[ODS] Support region names and constraints

    Similar to arguments and results, now we require region definition in ops to
    be specified as a DAG expression with the 'region' operator. This way we can
    specify the constraints for each region and optionally give the region a name.

    Two kinds of region constraints are added, one allowing any region, and the
    other requires a certain number of blocks.

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PiperOrigin-RevId: 250790211