message: Detect unknown opcodes
diff: @@ -67,7 +67,9 @@ function evalInstr(instr: bril.Instruction, env: Env): bril.Ident | null {
  // Check that we have the right number of arguments.
  if (instr.op !== "const") {
  let count = argCounts[instr.op];
  - if (count !== null) {
  + if (count === undefined) {
  + throw "unknown opcode " + instr.op;
  + } else if (count !== null) {
  checkArgs(instr, count);
  }
  }
  