[llvm] [SLP] SLP's copyable elements based upon Main/Alt operations. (PR #124242)

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

It must be rebased. Plus, I'm currently working on scheduling reworking, which, most probably, won't require scheduling model update in this patch, and it can be dropped.
Also, I mentioned that current approach misses some corner cases, like non-instructions, phis, etc. It should be addressed

https://github.com/llvm/llvm-project/pull/124242