fix(ssa): accept tainted arguments to no_predicates calls in flatten_post_check

[asterite]

Problem

Compiling contracts in Aztec-Packages (e.g. token_contract) with a debug-built compiler aborts with 11 SSA validation errors:

--- The SSA failed to validate after 'Flattening': Set the NOIR_SHOW_INVALID_SSA env var to see the SSA.
error: Value v22018 escapes `enable_side_effects` in function __aztec_nr_internals___recurse_subtract_balance

The flatten_post_check validator (debug-assertions only, added in #13084) did not account for the enable_side_effects u1 1 wrapper that flattening itself emits around calls to #[no_predicates] functions. Aztec contracts hit this constantly: oracle (Brillig) results fetched under a branch predicate — e.g. get_public_keys_and_partial_address inside conditionally-executed note logic — are passed to #[no_predicates] poseidon2 hash helpers:

enable_side_effects v5238
v22010, ..., v22017 = call f118(v21989)   // Brillig call → results tainted by v5238
v22018 = make_array [v22011, v22012]
enable_side_effects u1 1                  // no_predicates call wrapper
v22019 = call f147(v22018, ...)           // ← flagged: "v22018 escapes"

The escape is deliberate and safe: the callee runs unpredicated by design, and a pure callee cannot be over-constrained by a disabled-branch value. This was a false positive in the validator, not a miscompilation — release builds are unaffected since the check is #[cfg(debug_assertions)].

Minimal reproduction (fails to compile with a debug nargo before this fix):

#[no_predicates]
fn hash(input: [Field; 2]) -> Field {
    std::hash::poseidon2_permutation([input[0], input[1], 0, 0])[0]
}

unconstrained fn get_values(x: Field) -> (Field, Field) {
    (x + 1, x + 2)
}

fn main(c: bool, x: Field) -> pub Field {
    let mut r = 0;
    if c {
        // Safety: test
        let (a, b) = unsafe { get_values(x) };
        r = hash([a, b]);
    }
    r
}

Solution

Exempt argument uses at no_predicates call sites from the escape check. The exemption only skips the violation: the call's results still inherit the argument's predicate and stay tracked until guarded, since they are arbitrary on the disabled branch.

Tests:

• accepts_tainted_argument_to_no_predicates_call — the distilled Aztec shape above (fails without the fix).
• rejects_unguarded_result_of_no_predicates_call — same SSA minus the result guard, pinning that the exemption does not leak to the call's results.

Verified that the token_contract in Aztec-Packages compiles with zero validation failures after this change (previously aborted with 11 errors).

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