[mlir][dataflow] Remove Lattice::isUninitialized().

Currently, for sparse analyses, we always store a `Optional<ValueT>` in each lattice element. When it's `None`, we consider the lattice element as `uninitialized`.

However:

* Not all lattices have an `uninitialized` state. For example, `Executable` and `PredecessorState` have default values so they are always initialized.

* In dense analyses, we don't have the concept of an `uninitialized` state.

Given these inconsistencies, this patch removes `Lattice::isUninitialized()`. Individual analysis states are now default-constructed. If the default state of an analysis can be considered as "uninitialized" then this analysis should implement the following logic:

* Special join rule: `join(uninitialized, any) == any`.

* Special bail out logic: if any of the input states is uninitialized, exit the transfer function early.

Depends On D132086

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D132800