ASSERT_SLEEPABLE(9): Micro-optimize this a little bit.

This convinces gcc to do less -- make a smaller stack frame, compute
fewer conditional moves in favour of predicted-not-taken branches --
in the fast path where we are sleepable as the caller expects.

Wasn't able to convince it to do the ncsw loop with a
predicted-not-taken branch, but let's leave the __predict_false in
there anyway because it's still a good prediction.

(riastradh)