s/// in boolean context: simplify return value

s/// in boolean context: simplify return value

Normally s/// returns a count of the number of iterations, but
as an optimisation, in boolean context it returns PL_sv_yes/PL_sv_zero
instead. But in the places where it decides which immortal var to return,
the number of iterations is always > 0, so PL_sv_zero never gets returned.
So skip testing whether iters > 0 and always just return PL_sv_yes.

(In non-boolean scalar context, it still returns the iteration count as
before.)