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

[Philip Reames via llvm-commits]

preames wrote:

I don't believe this is possible to test on a target without scalable vector support.  I tried, but the costs returned are all invalid, and thus we refuse to vectorize entirely.  (Which seems like the correct result)

Would folks rather I land this with the code path untested, or convert the check to an assertion?  

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