[llvm] [LV] Add initial legality checks for loops with unbound loads. (PR #152422)

[Luke Lau via llvm-commits]

lukel97 wrote:

> I would like to know why the vectorization of PotentiallyFaultingLoads was directly rejected. I think this is unreasonable. After all, the following code
> 
> ```
> void bar(float *A, float* B, float K, int start, int end) {
>   for (int i = start; i < end; ++i)
>     A[i] *= B[i] + K;
> }
> ```
> 
> The "load" operation is not necessarily valid, but it can also be vectorized.

We vectorize the loop above because A[i] and B[i] will at least exhibit the same observable trapping when vectorized, so they're considered dereferenceable.

PotentiallyFaultingLoads contains loads like this:

```
for (int i = 0; i < n; i++) {
  if (A[i] == 0)
    break;
  A[i]++;
}
```

If A[2] == 0 but A[3] isn't dereferenceable, the original scalar loop won't trap.

If we vectorize this with VF=4 we would trap trying to access A[3]. We specifically need fault-only-first instructions to vectorize this safely, and the support for those hasn't landed yet.

https://github.com/llvm/llvm-project/pull/152422