[MachinePipeliner] Remove isLoopCarriedDep and use DDG (#174394) This patch completely removes `isLoopCarriedDep`, which was used previously to identify loop-carried dependencies in the DAG. Now that we have the DDG representation, this special handling is no longer necessary. Simply replacing its usage with the DDG causes several tests to fail, since cycle detection takes some of the validation-only edges in the DDG into account. To address this, this patch introduces extra edges in the DDG, which are used only for cycle detection and not for other parts of the pass (e.g., scheduling). The extra edges are determined to preserve the existing behavior of the pass as closely as possible, which makes the predicates for adding them somewhat complex. Split off from #135148, and the final patch in the series for #135148
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