[llvm] [LV] Consider whether vscale is a known power of two for iteration check (PR #144963)

[Philip Reames via llvm-commits]

preames wrote:

I had just pushed a change which turned this into an assert instead, but am happy to go with whatever reviewers prefer.  Honestly, I'm wondering if we should just update LangRef to require vscale to be a power of two.  If a target comes along for which this property doesn't hold, we could just relax it then.  

https://github.com/llvm/llvm-project/pull/144963