For loops with symbolic values: SSA threading across iterations

[FrancoGiachetta]

Problem

For loops currently cannot handle symbolic values that are reassigned across iterations.

Symbolic values use pure SSA — assign_local only updates a Rust-side Vec, but the MLIR back-edge (cf::br with empty args &[]) has no mechanism to carry the updated SSA value from the end of one iteration to the start of the next.

Example that should work

df main() i32 {
    sym x: expr<i32>;
    let e: expr<i32> = x;
    for i in 0..10 {
        e = e + 1;
    }
    return eval(e, x, 0);
}