CombDataflow op interface should not exist in the HW dialect

[prithayan]

The CombDataflow op interface is being used only in the FIRRTL dialect, CheckCombLoops pass, resolve the downstream dependencies to ensure the interface can be defined in the FIRRTL dialect. Related to: https://github.com/llvm/circt/pull/7195/files

[mortbopet]

Drive-by comment: Alternatively, something like CheckCombLoops seems like a perfectly applicable, relevant and generally very useful pass to have at a core-dialect level... so perhaps it isn't the interface that needs to move (may be it needs to move to comb), but instead the pass that needs to be made more generic.

[sequencer]

but instead the pass that needs to be made more generic.

Agreed, #8610 also illustrated this. If some Op is not handled in firrtl, and delegate to hw, then CheckCombLoops goes wrong.