If the server is compiled WITH_WSREP=OFF, we should avoid evaluating
conditions on a global variable that is constant.

WSREP_ON_: Renamed from WSREP_ON. Defined only WITH_WSREP=ON.

WSREP_ON: Defined as unlikely(WSREP_ON_).

wsrep_on(): Defined as WSREP_ON && wsrep_service->wsrep_on_func().

The reason why we have wsrep_on() at all is that the macro WSREP(thd)
depends on the definition of THD, and that is intentionally an opaque
data type for InnoDB. So, we cannot avoid invoking wsrep_on(), but
we can evaluate the less expensive condition WSREP_ON before calling
the function.