chore: improve performance of mpure_intro and mvcgen by avoiding whnfD (#12165)
New measurements:
```
goal_10: 181.910200 ms, kernel: 37.241050 ms
goal_20: 386.540215 ms, kernel: 83.497428 ms
goal_30: 648.282057 ms, kernel: 117.038447 ms
goal_40: 946.733191 ms, kernel: 168.369124 ms
goal_50: 1325.846873 ms, kernel: 223.838786 ms
goal_60: 1734.175705 ms, kernel: 285.594486 ms
goal_70: 2199.522317 ms, kernel: 351.659865 ms
goal_80: 2700.077802 ms, kernel: 428.303337 ms
goal_90: 3260.446641 ms, kernel: 515.976499 ms
goal_100: 3865.503733 ms, kernel: 600.229962 ms
```
Previously, goal_100 took 7.8s.
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