The rule for analysing the uniformity of break if statements (here) currently throws away the value node V representing the uniformity of the expression. This leads the following shader to be incorrectly accepted as uniform under the analysis in the language spec, making it unsound:

@compute @workgroup_size(16)
fn main(@builtin(local_invocation_index) lid: u32) {
  loop {
      workgroupBarrier();
      continuing {
        break if lid < 0;
      }
    }

}

It seems that the rule for break if statements should add an edge from CF' to V to signify that the uniformity going forward depends on the uniformity of V. This appears to be the approach that tint takes which leads it to correctly identify this example as non-uniform.