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

[Luke Lau via llvm-commits]

lukel97 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

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