[mlir][dataflow] Unify dependency management in AnalysisState.

In the MLIR dataflow analysis framework, when an `AnalysisState` is updated, it's dependents are enqueued to be visited.

Currently, there are two ways dependents are managed:

* `AnalysisState::dependents` stores a list of dependents. `DataFlowSolver::propagateIfChanged()` reads this list and enqueues them to the worklist.

* `AnalysisState::onUpdate()` allows custom logic to enqueue more to the worklist. This is called by `DataFlowSolver::propagateIfChanged()`.

This cleanup diff consolidates the two into `AnalysisState::onUpdate()`. This way, `DataFlowSolver` does not need to know the detail about `AnalysisState::dependents`, and the logic of dependency management is entirely handled by `AnalysisState`.

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D154170