[llvm] [SLP] Vectorize non-power-of-2 ops with padding. (PR #77790)

[Florian Hahn via llvm-commits]

fhahn wrote:

> Most of this newly added stuff, like padding etc., is not needed, if everything is implemented correctly. This is not a complete solution, but just a small hack to enable the feature, which may cause issues in future and adds extra burden to maintaining and removing it later.

Agreed that the padding isn't needed, did you see 0ddbdd3052e694eedef6bccce3b17f92dff7add7 mentioned in one of  my previous comment that removes all the padding stuff? 

There are still a few things that can be cleaned up there, but with that removed, the changes mostly boil down to disabling shuffling/reordering, as most of that code isn't able to handle non-power-of-2 vectors yet. As support is added for those, the restrictions could be loosened gradually.

https://github.com/llvm/llvm-project/pull/77790