[llvm] [LoopInterchange] Ignore the cost-model, force interchange if legal (PR #148858)

[Ryotaro Kasuga via llvm-commits]

https://github.com/kasuga-fj commented:

orce interchange can occur odd behavior in some cases. Can you add a test to show it? Here is an example: 

```c
int A[4][4][4];
for (int i = 0; i < 4; i++)
  for (int j = 1; j < 4; j++)
    for (int k = 0; k < 4; k++)
      A[3-i][j-1][k] = A[i][j][k];
```

IIUIC, the following scenario happens due to the bubble-sort based heuristic:

1. Try to interchange the 2nd loop (j-loop) and 3rd loop (k-loop). It is legal, so they are interchanged.
2. Try to interchange the 1st loop (i-loop) and 2nd loop (k-loop, because the j-loop and k-loop were interchanged). It is illegal, so they are NOT interchanged.
3. Try to interchange the 2nd loop (k-loop) and 3rd loop (j-loop). It is legal indeed, so they are interchanged again.

Consequently, the first interchange is reverted, and the final loop order remains the same as the original. I believe this is acceptable, considering that the force interchange option is intended for testing purposes. However, it is worth clarifying this behavior with an example.

https://github.com/llvm/llvm-project/pull/148858