message: brili: Permit "unassignment" in phi-nodes
  This allows the execution of dead phi-nodes that take their value from
  only some in-edges. These instructions must be dead, but it's nice to be
  able to execute them before cleaning them up.
diff: @@ -651,10 +651,12 @@ function evalInstr(instr: bril.Instruction, state: State): Action {
  }
  let idx = labels.indexOf(state.lastlabel);
  if (idx === -1) {
  - throw error(`phi node had unhandled last label ${state.lastlabel}`);
  - }
  + // Last label not handled. Leave uninitialized.
  + state.env.delete(instr.dest);
  + } else {
  let val = getArgument(instr, state.env, idx);
  state.env.set(instr.dest, val);
  + }
  return NEXT;
  }
  