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

[Luke Lau via llvm-commits]

lukel97 wrote:

> Nope, I continue to get "remark: :0:0: UserVF ignored because of invalid costs.".

Fair enough, I don't have any better ideas. I'm fine to convert it to an assertion

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