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

[Julian Brown via llvm-commits]

jtb20 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.

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