Unnest compound mode search loops and clean up inter search (#5343)

This change includes the following cleanups 
- Unnest JMVD scale factor, CWP, and RefineMV loops in
handle_compound_inter_prediction to avoid redundant motion search and
skip invalid tool combinations. The new search order is 1) base mode, 2)
refine MV, 3) CWP, 4) JMVD scale. This search order is intended to allow
potential early termination in the future
- Remove unused variables, unnecessary checks, redundant mbmi field
re-assignments, and move the update of common variable outside of search
loops.
- Use reset_inter_mode_info helper to unify mode_info field resets

STATS_CHANGED

Results tested with RA 33 frames based on 18a0096

```
    1) Speed 0
      +------------+-------+-------+-------+-------+-------+-------+
      | Class      |     Y |    Cb |    Cr |  wAvg |  Enc% |  Dec% |
      +------------+-------+-------+-------+-------+-------+-------+
      | A1         | -0.01 | +0.02 | +0.12 | -0.01 | 100.8 |  99.4 |
      | A2         | -0.00 | +0.06 | +0.01 | +0.00 | 100.2 |  99.4 |
      +------------+-------+-------+-------+-------+-------+-------+
    
    2) Speed 1
      +------------+-------+-------+-------+-------+-------+-------+
      | Class      |     Y |    Cb |    Cr |  wAvg |  Enc% |  Dec% |
      +------------+-------+-------+-------+-------+-------+-------+
      | A1         | +0.00 | +0.02 | +0.03 | +0.01 | 100.9 | 101.4 |
      | A2         | +0.01 | +0.01 | -0.07 | +0.01 | 100.7 | 100.5 |
      +------------+-------+-------+-------+-------+-------+-------+
    
    3) Speed 2
      +------------+-------+-------+-------+-------+-------+-------+
      | Class      |     Y |    Cb |    Cr |  wAvg |  Enc% |  Dec% |
      +------------+-------+-------+-------+-------+-------+-------+
      | A1         | -0.01 | +0.00 | -0.02 | -0.01 | 101.0 | 101.6 |
      | A2         | +0.01 | +0.02 | -0.01 | +0.01 | 100.5 | 100.7 |
      +------------+-------+-------+-------+-------+-------+-------+
```

Author: kslu-aom