ZJIT: Count fallback reasons for set/get/definedivar (#15324)

lobsters:

```
Top-4 setivar fallback reasons (100.0% of total 7,789,008):
  shape_transition: 6,074,085 (78.0%)
   not_monomorphic: 1,484,013 (19.1%)
      not_t_object:   172,629 ( 2.2%)
       too_complex:    58,281 ( 0.7%)
Top-3 getivar fallback reasons (100.0% of total 9,348,832):
     not_t_object: 4,658,833 (49.8%)
  not_monomorphic: 4,542,316 (48.6%)
      too_complex:   147,683 ( 1.6%)
Top-3 definedivar fallback reasons (100.0% of total 366,383):
  not_monomorphic: 361,389 (98.6%)
      too_complex:   3,062 ( 0.8%)
     not_t_object:   1,932 ( 0.5%)
```

railsbench:

```
Top-3 setivar fallback reasons (100.0% of total 15,119,057):
  shape_transition: 13,760,763 (91.0%)
   not_monomorphic:    982,368 ( 6.5%)
      not_t_object:    375,926 ( 2.5%)
Top-2 getivar fallback reasons (100.0% of total 14,438,747):
     not_t_object: 7,643,870 (52.9%)
  not_monomorphic: 6,794,877 (47.1%)
Top-2 definedivar fallback reasons (100.0% of total 209,613):
  not_monomorphic: 209,526 (100.0%)
     not_t_object:      87 ( 0.0%)
```

shipit:

```
Top-3 setivar fallback reasons (100.0% of total 14,516,254):
  shape_transition: 8,613,512 (59.3%)
   not_monomorphic: 5,761,398 (39.7%)
      not_t_object:   141,344 ( 1.0%)
Top-2 getivar fallback reasons (100.0% of total 21,016,444):
  not_monomorphic: 11,313,482 (53.8%)
     not_t_object:  9,702,962 (46.2%)
Top-2 definedivar fallback reasons (100.0% of total 290,382):
  not_monomorphic: 287,755 (99.1%)
     not_t_object:   2,627 ( 0.9%)
```

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