Add and test `[[assume]]` for goto

[kammce]

We'd do this:

  while (true) {
    auto const* instruction_handler = jump_table[*instruction_ptr];
    instruction_ptr++;
    [[assume(instruction_handler != nullptr]];
    goto* instruction_handler;
    // etc.

and we need to see if this nets us any benefit. I remember, in the past, seeing goto perform a nullptr check prior to performing the goto. The hope is that the assume will guide GCC to skip the nullptr check.