[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?
> 
> +1, I'm thinking that the assertion would just be `assert(TTI->isVScaleKnownPowerOf2())` and it would replace that code

Thinking about this now, the problem may be because the target also has to support masked loads and stores. I'm going to see what happens if I can vectorise a loop with memory ops.

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