[llvm] [SLP][NFC] Avoid the getRdxOperand hack (PR #148672)

[Piotr Fusik via llvm-commits]

pfusik wrote:

> I would keep the original functions. Maybe, do some early checks for index 0 and later checks for index 1, but still keep the existing abstractions.

The comment says "TODO: Change the code and data structures to handle this without a hack."
I'm doing exactly this. How to keep these functions in place and why?

https://github.com/llvm/llvm-project/pull/148672