message: fix to primary_instruction_of_move_result
  Summary:
  Skip over non-opcodes, in particular, trivial fallthrough branch targets.
  (This caused crashes when running CopyPropagation early as part of CSE, configured higher up in the stack.)
diff: @@ -1285,7 +1285,12 @@ cfg::InstructionIterator ControlFlowGraph::primary_instruction_of_move_result(
  always_assert(insn->has_move_result_any());
  return res;
  } else {
  - auto res = std::prev(it.unwrap());
  + auto res = it.unwrap();
  + do {
  + res = std::prev(res);
  + // skip over non-opcodes (in particular, trivial fallthrough branch
  + // targets)
  + } while (res->type != MFLOW_OPCODE);
  auto insn = res->insn;
  always_assert(insn->has_move_result_any());
  return block->to_cfg_instruction_iterator(res);
  