Before this commit, the dla::Step depended on InterferingInfo, but this
was superfluous.
This commit drops the dependency and enables the Step to run before
ComputeNonInterferingComponents.
This also avoids the need to run ComputeNonInterferingComponents twice
(one before and one after DeduplicateUnionFields), so it can only run
once.
The step now collapse parents with their single child if they are
indistinguishable, i.e. if the parent has only that single child, at
offset zero, and their size is the same.
Alessandro Di Federico