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

[Piotr Fusik via llvm-commits]

pfusik wrote:

> Yes, but you did not fix it, just moved to another place. Instead, would be good just to build the list of operands (actual operands, excluding true value) and just return the corresponding operand without adjusting the index

The list of operands is now `I1` and `I2`. I find it much cleaner than a reversed sequence and substituting an index.
I don't know how to improve it.

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