[mlir][CFGToSCF] Add interface changes for downstream projects

This is a follow-up to https://reviews.llvm.org/D156889

Downstream projects may have more complicated ops than the control flow ops up

[mlir][CFGToSCF] Add interface changes for downstream projects

This is a follow-up to https://reviews.llvm.org/D156889

Downstream projects may have more complicated ops than the control flow ops upstream and therefore need a more powerful interface to support the lifting process. Use cases include the propagation of (inherent) metadata that was previously on the control flow ops and now needs to be lifted to structured control flow ops.
Since the lifting process is inherently non-local in respect to the function-body, we require stronger guarantees from the interface.

This patch therefore makes two changes to the interface:
* Passes the terminator that is being replaced to `createStructuredBranchRegionTerminatorOp`
* Adds as precondition to `createCFGSwitchOp` that its predecessors are already correctly established

Asserts have been added to verify these were it makes sense and to correctly state intent. I have not added tests purely because testing preconditions like these is not really feasible (and incredibly specific).

Differential Revision: https://reviews.llvm.org/D157981

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[mlir][NFC] Expose the `CFGToSCFInterface` for the `ControlFlowToSCF` pass

This is useful for any downstream users who may just want to use slightly different ops than the pass or need to take into

[mlir][NFC] Expose the `CFGToSCFInterface` for the `ControlFlowToSCF` pass

This is useful for any downstream users who may just want to use slightly different ops than the pass or need to take into account other ops in the input while still dealing with ControlFlow ops for the most part.
This also helps writing test implementations for `transformCFGToSCF`.

Since the implementation is now public, comments noting the post conditions of the methods have been added to ensure downstream users can stay compatible with the existing implementation.

Differential Revision: https://reviews.llvm.org/D157971

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[mlir][cf] Add ControlFlow to SCF lifting pass

Structured control flow ops have proven very useful for many transformations doing analysis on conditional flow and loops. Doing these transformations

[mlir][cf] Add ControlFlow to SCF lifting pass

Structured control flow ops have proven very useful for many transformations doing analysis on conditional flow and loops. Doing these transformations on CFGs requires repeated analysis of the IR possibly leading to more complicated or less capable implementations. With structured control flow, a lot of the information is already present in the structure.

This patch therefore adds a transformation making it possible to lift arbitrary control flow graphs to structured control flow operations. The algorithm used is outlined in https://dl.acm.org/doi/10.1145/2693261. The complexity in implementing the algorithm was mostly spent correctly handling block arguments in MLIR (the paper only addresses the control flow graph part of it).

Note that the transformation has been implemented fully generically and does not depend on any dialect. An interface implemented by the caller is used to construct any operation necessary for the transformation, making it possible to create an interface implementation purpose fit for ones IR.

For the purpose of testing and due to likely being a very common scenario, this patch adds an interface implementation lifting the control flow dialect to the SCF dialect.
Note the use of the word "lifting". Unlike other conversion passes, this pass is not 100% guaranteed to convert all ControlFlow ops.
Only if the input region being transformed contains a single kind of return-like operations is it guaranteed to replace all control flow ops. If that is not the case, exactly one control flow op will remain branching to regions terminating with a given return-like operation (e.g. one region terminates with `llvm.return` the other with `llvm.unreachable`).

Differential Revision: https://reviews.llvm.org/D156889

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