Counter in slice iter fails to eliminate bounds check

The following code (somewhat extracted from #126425) includes an unreachable bounds check panic

pub fn getnonzero(buf: &[u8; 128]) -> &[u8] {
    let mut curr = 0;
    for n in buf.iter() {
        if *n == 0 { break; }
        curr += 1;
    }
    &buf[..curr]
}

https://godbolt.org/z/vzooGexnj

This bounds check can be eliminated: Alive2

This also occurs with reverse iteration (like in the original) and iterating 0..buf.len() instead of direct slice iteration. Lowering the slice length to the point where the loop is fully unrolled does eliminate this, but the exact number depends on the specific code.

Compile the provided getnonzero reproducer and inspect its generated output through Godbolt, comparing direct, reverse, and range iteration variants. Trace the compiler optimization path responsible for the bounds check and validate changes against the Alive2 example. Done means the unreachable check is eliminated for the reported cases without regressing other cases.