arybase.xs: Always check the op type in ck_*

arybase.xs: Always check the op type in ck_*

Sometimes the previous check function will have replaced the op
with one of a different type.  In that case, the rest of arybase’s
check function does not apply and can even cause a crash for ops
with no children (e.g., $x=wait is optimised down to a simple wait
op with no children and with op_targ pointing to $x, if it is
lexical).