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

[David Sherwood via llvm-commits]

david-arm wrote:

If it really is impossible to test, then surely we should just delete the entire block of code to avoid having to maintain something that's impossible to test? What do you think @fhahn?

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