This PR upstreams `GotoOp`. It moves some tests from the `goto` test
file to the `label` test file, and adds verify logic to `FuncOp`. The
gotosSolver, required for lowering, will be implemented in a future PR.
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
