I also tried this

  bool is_divisible_by_15(int x) {
      return x % 3 == 0 && x % 5 == 0;
  }

  bool is_divisible_by_15_optimal(int x) {
      return x % 15 == 0;
  }

is_divisible_by_15 still has a branch, while is_divisible_by_15_optimal does not

  is_divisible_by_15(int):
        imul    eax, edi, -1431655765
        add     eax, 715827882
        cmp     eax, 1431655764
        jbe     .LBB0_2
        xor     eax, eax
        ret
  .LBB0_2:
        imul    eax, edi, -858993459
        add     eax, 429496729
        cmp     eax, 858993459
        setb    al
        ret

  is_divisible_by_15_optimal(int):
        imul    eax, edi, -286331153
        add     eax, 143165576
        cmp     eax, 286331153
        setb    al
        ret