[llvm] [AArch64] Allow unrolling of scalar epilogue loops (PR #151164)

[John Brawn via llvm-commits]

john-brawn-arm wrote:

> > Overall, I'm pretty confused about when is.vectorized is emitted, and when it isn't: can you comment on whether LV should, in principle, emit is.vectorized for the scalar remainder?
> 
> Yes I think so, this is to avoid vectorizing again, e.g. when doing LTO

Yes, in LoopVectorize.cpp where it sets this it says
```
  // Mark the loop as already vectorized to avoid vectorizing again.
  Hints.setAlreadyVectorized();
```


https://github.com/llvm/llvm-project/pull/151164