There are many cases where we would benefit from a constant vector.

One example we found here:
https://github.com/openjdk/jdk/pull/22856 / JDK-8346664

        for (int i = 0; i < count; i++) {
            dst[i] = src[i] * (i & 7);
        }

Because

    // Does not vectorize: due to sum-under-mask optimization.
    // (i+0) & 7, (i+1) & 7 ... (i+8) & 7 .... -> PopulateIndex
    // becomes
    // (i+0) & 7, (i+1) & 7 ... (i+0) & 7 .... -> pattern broken

We could just introduce some constants vector, and add this instead of the iota_indices.

That would allow us to generalize it to any i+c0, i+c1, i+c2, ....