[llvm] [SLP] Sort PHIs by ExtractElements when relevant (PR #131229)

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

> I'm still studying the code, but I'm wondering whether BoUpSLP::getReorderingData should be handling this case? It has its own phi node sorting, and indeed seems to be properly detecting identity order for the missed optimisation case here:
> 
> ```
>     if (IsIdentityOrder(ResOrder))
>       return std::nullopt; // No need to reorder.
> ```
> 
> but, the "no need to reorder" case fires, and so the shuffles are left as-is rather than being removed.


Generally speaking, implementing it in getReorderingData should be enough, but having it here as the first step also should be fine

https://github.com/llvm/llvm-project/pull/131229