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

[Alexey Bataev via llvm-commits]

alexey-bataev 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.

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