LoopFusionUtils CL 2/n: Factor out and generalize slice union computation.

    *) Factors slice union computation out of LoopFusion into Analysis/Utils (where other iteration slice utilities exist).
    *) Generalizes slice union computation to take the union of slices computed on all loads/stores pairs between source and destination loop nests.
    *) Fixes a bug in FlatAffineConstraints::addSliceBounds where redundant constraints were added.
    *) Takes care of a TODO to expose FlatAffineConstraints::mergeAndAlignIds as a public method.

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PiperOrigin-RevId: 250561529