message: Compile `br`
diff: @@ -212,6 +212,13 @@ fn compile_inst(
  let block = *blocks.get(&labels[0]).unwrap();
  builder.ins().jump(block, &[]);
  }
  + bril::EffectOps::Branch => {
  + let arg = builder.use_var(*vars.get(&args[0]).unwrap());
  + let true_block = *blocks.get(&labels[0]).unwrap();
  + let false_block = *blocks.get(&labels[1]).unwrap();
  + builder.ins().brnz(arg, true_block, &[]);
  + builder.ins().jump(false_block, &[]);
  + }
  _ => todo!()
  }
  }
  