[llvm] [NVPTX] Only run LowerUnreachable when necessary (PR #109868)

[Artem Belevich via llvm-commits]

Artem-B wrote:

If we know which ptxas version no longer needs `exit`, then we may use PTX version that's only supported by that version of ptxas (or the driver), or newer. This guarantees that it will never be compileable by the older ptxas with the bug.

```
  bool hasPTXASUnreachableBug() const {
    return SmVersion < 70 && PTXVersion <= 74;
  }
```

With that in mind, this condition should be changed appropriately. Assuming that it's the latest public CUDA release that has the bug fixed, we should only mark PTX ISA 8.5 or newer as safe.  https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#release-notes




https://github.com/llvm/llvm-project/pull/109868