[EpilogTailFolding] Create CostModel instance for tail-folded epilogue

We need to create a CM instance dedicated for tail-folded epilogue to make separation between the costs calculated for the unpredicated vector main loop and the predicated (tail-folded) vector epilogue.

What we need to do here:

- Create an instance of CM for tail-folded epilogue.
- Populate CM functions to consider the new option for epilogue lowering.
- Keep that CM instance (inactive). It should NOT affect the final output. We are still NOT generating vectorized predicated epilogue.
- Add tests