[mlir][CF] Split `cf-to-llvm` from `func-to-llvm` (#120580)

Do not run `cf-to-llvm` as part of `func-to-llvm`. This commit fixes
https://github.com/llvm/llvm-project/issues/70982.

This commit changes the way how `func.func` ops are lowered to LLVM.
Previously, the signature of the entire region (i.e., entry block and
all other blocks in the `func.func` op) was converted as part of the
`func.func` lowering pattern.

Now, only the entry block is converted. The remaining block signatures
are converted together with `cf.br` and `cf.cond_br` as part of
`cf-to-llvm`. All unstructured control flow is not converted as part of
a single pass (`cf-to-llvm`). `func-to-llvm` no longer deals with
unstructured control flow.

Also add more test cases for control flow dialect ops.

Note: This PR is in preparation of #120431, which adds an additional
GPU-specific lowering for `cf.assert`. This was a problem because
`cf.assert` used to be converted as part of `func-to-llvm`.

Note for LLVM integration: If you see failures, add
`-convert-cf-to-llvm` to your pass pipeline.