If we do, we generate code that statically refers to the array type
`void[]`, which is not a valid type.

With the following commit, the optimizer will have enough knowledge
to enter in this case when optimizing `ClassTag`s, resulting in
ClassDef checking errors after the optimizer.

`ClassType`s and `ArrayType`s now carry a `nullable: Boolean` flag.
When `nullable` is false, the type does not admit `null` values.
We also introduce `AnyNotNullType` to be the non-nullable variant
of `AnyType`.

This way, every non-void type `tpe` has a non-nullable variant,
which we can get with `tpe.toNonNullable`.

There are a few sources of non-nullable values, such as `New`
nodes, and, most importantly, `This` nodes.

Since non-nullable reference types have no corresponding `TypeRef`,
they cannot appear in the signature of methods. They only live
locally within method bodies.

---

The optimizer already had dedicated tracking of non-nullable
`PreTransform`s. We now replace that tracking by the actual `tpe`
of the trees.

In order to be type-preserving, we must now insert most `Cast`s,
to non-nullable types. Before, at the IR level everything became
nullable again, so even if the optimizer had determined that a tree
could not be null, that did not influence the produced IR.

On the plus side, since IR nodes track their nullability, we do not
need `AssumeNotNull` anymore. The `Cast`s to non-nullable types
achieve the same result in a more consistent way.

In fact, the new non-nullable types allow the optimizer to better
keep track of nullability, resulting in fewer `$n` calls to check
for nulls. This is the main source of code side reduction.

---

Since we now have non-nullable reference types, we change
`IsInstanceOf` to require non-nullable test types. This makes more
sense, since `IsInstanceOf` always answers `false` when the value
is `null`.

---

We introduce deserialization hacks to:

* adapt the type of `This` nodes, and
* adapt the test type of `IsInstanceOf` nodes.

In this commit, we do not change the compiler nor the hard-coded IR
of `jl.Object` yet, in order to test the deserialization hacks.

For `This` nodes and `IsInstanceOf` nodes.

We only active the relevant deserialization hacks when reading IR
from before 1.17.