This PR introduces the `LabelOp`, which is required for implementing
`GotoOp` lowering in the future.
Lowering to LLVM IR is **not** included in this patch, since it depends
on the upcoming `GotoSolver`.
The `GotoSolver` traverses the function body, and if it finds a
`LabelOp` without a matching `GotoOp`, it erases the label.
This means our implementation differs from the classic codegen approach,
where labels may be retained even if unused.
Example:
https://godbolt.org/z/37Mvr4MMr
4b112d23a5