[llvm] [LV] Clarify code in isPredicatedInst (NFC) (PR #134251)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> I am not sure which aspect this clarifies?

See following comment:

```cpp
  // All that remain are instructions with side-effects originally executed in
  // the loop unconditionally, but now execute under a tail-fold mask (only)
  // having at least one active lane (the first). If the side-effects of the
  // instruction are invariant, executing it w/o (the tail-folding) mask is safe
  // - it will cause the same side-effects as when masked.
```

We make it clear that we execute under a tail-fold mask (only). Also, this is a preparatory step for https://github.com/llvm/llvm-project/pull/134261.

https://github.com/llvm/llvm-project/pull/134251