[Unity] Implement RemoveUnusedOutputs transform
This commit implements `relax.transform.RemoveUnusedOutputs`, an
extension of the current `DeadCodeElimination` pass. If an internal
callee produces multiple outputs, but only some of those outputs are
used, the callee can be rewritten to only produce the outputs that are
used. This is intended to allow more aggressive DCE in the callee, as
reducing the set of outputs can result in intermediate computations
becoming unused.
Eric Lunderberg
d52a9bf388