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

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM and reverse ping. I stumbled across some tail folding code in the loop vectorizer which is complicated by the fact that we allow non-power-of-2 vscales.

To the best of my knowledge I am not aware of any VLA ISAs, current or planned, that allow for non-power-of-two vector sizes. 

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