[llvm] [SLP] NFC. Refactor getSameOpcode and reduce for loop iterations. (PR #122241)

[Mikael Holmén via llvm-commits]

mikaelholmen wrote:

Hi @HanKuanChen and @alexey-bataev ,

The following starts crashing with this "NFC" patch:
```
opt -passes="slp-vectorizer" bbi-103405.ll -S -o /dev/null -mtriple=thumb7 -mcpu=swift
```
[bbi-103405.ll.gz](https://github.com/user-attachments/files/18573264/bbi-103405.ll.gz)


https://github.com/llvm/llvm-project/pull/122241