sim: ppc: collapse is_valid switch table more

Instead of writing:
  case 1:
    return 1;
  case 2:
    return 1;
  ...etc...

Output a single return so we get:
  case 1:
  case 2:
  case ...
    return 1;

This saves ~100 lines of code.  Hopefully the compiler was already
smart enough to optimize to the same code, but if not, this probably
helps there too :).