[llvm] [LoopVectorize] Vectorize select-cmp reduction pattern for increasing integer induction variable (PR #67812)

[Mel Chen via llvm-commits]

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Mel-Chen wrote:

No, we must retain `llvm/test/Transforms/LoopVectorize/select-min-index.ll`. `select-min-index.ll` demonstrates another semantics: minmax with index. This semantics is similar to FindLastIV but different.
```
int min = start_min;
int idx = start_idx;
for (int i = 0; i < n; i++)
  if (min > a[i]) {
    min = a[i];
    idx = i;
  }

// live-out: idx
```

https://github.com/llvm/llvm-project/pull/67812