[llvm] [LAA] Fix off-by-EltSize in negative-step deref bounds check (PR #211964)

[Andrei Elovikov via llvm-commits]

https://github.com/eas commented:

This is way out of my comfort zone, so I won't be approving, but I also don't see anything wrong with this patch.

https://github.com/llvm/llvm-project/pull/211964