[llvm] [LangRef] Require that vscale be a power of two (PR #145098)

[Paul Walker via llvm-commits]

https://github.com/paulwalker-arm approved this pull request.

I agree with removing the redundant TTI interface but otherwise this feels like a retrograde step given we've already put the work in. I feel sorry for whoever needs to walk this back, especially having to audit existing IR and IR creators to establish if their use of `vscale` comes with some now hidden power-of-two requirement.  That said, the decision is to the benefit of AArch64 so I'll not fight the consensus.

Does this warrant an RFC (or rather, notification of intent) just in case there's an out-of-tree toolchain that relies on the current definition?

https://github.com/llvm/llvm-project/pull/145098