pub fn aggressive_inline_let(expr: &mut Expr) {
    let mut subbed_one = false;
    expr.transform_up(&mut |ref mut expr| {
        if subbed_one {
            return None;
        }
        if let Let {
            ref mut name,
            ref mut value,
            ref mut body,
        } = expr.kind
        {
            if !is_fusable_expr(value) || !only_used_in_zip(name, body) {
                return None;
            }
            let mut new_body = body.as_ref().clone();
            new_body.substitute(name, value);
            subbed_one = true;
            Some(new_body)
        } else {
            None
        }
    });
}
Aggressively inlines let statements in cases which allow loop fusion to fire. This inliner is
aggressive because it will replace identifiers which appear more than once after being defined.
However, the inliner will only fire if eventually, the inlined loop will be fused.
weld/src/optimizer/transforms/loop_fusion_2.rs