Since D13404, pendingToRealizedThreadIDsSelector will return the same thing when queried with a thin thread versus a thick thread.

As such, there's no longer a reason to check for both separately here.

Separately, I also got rid of the ternary at the end in favor of an early exit.

Depends on D13533

Careful reading of the code, as well as being tested with the rest of the stack, where I test all kinds of ways of creating a thread