Phase to desugar as patterns (`x @ subpattern`)

[W95Psp]

Some backends (e.g. F*) don't support as patterns, and reject them. For instance:

fn f(x: Option<u8>) -> u8 {
    match x {
        _binder @ Some(x) => x,
        _ => 0,
    }
}

We should have a phase that rewrites such matches. With the new phase @maximebuyse added that rewrites if let-guards, this comes for almost free: we could for instance rewrite binder @ Some(x) as binder if let Some(x) = binder.

[github-actions[bot]]

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[W95Psp]

Still relevant