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

[Philip Reames via llvm-commits]

preames wrote:

> The last time this was discussed the conclusion was that if we specify vscale must be a power-of-two then realistically we can never walk it back because it'll be too hard to find the places that rely on the assumption. The compromise was to change the definition of `vscale_range` to imply vscale is a power-of-two. In the looses sense `vscale_range(1,0)` says nothing more than it's a power-of-two. In my view the TTI hook should be removed.

I am increasingly thinking this was the wrong decision.  The only hardware we know of has power-of-two sizes, and keeping complexity for a potential future use case is generally undesirable.  Finding the places that assume power of two again (if ever needed) doesn't seem that hard - compared to implementing any reasonable complicated feature at least.  

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