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

[Paul Walker via llvm-commits]

paulwalker-arm 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.

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