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

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

As I said before, landing this alone causes perf regressions. I'm working on this, need to land couple patches to avoid this. I'm working on this feature for 3 years already (or more), most part of my previous patches are related to non-power-of-2 support.

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