pp_enter: calculate gimme earlier in XS branch

pp_enter: calculate gimme earlier in XS branch

My commit 801bbf618dc make it so that pp_entersub only calculates
gimme at the point its needed, to avoid wasting register resource.
However in n the XS branch it was a bit over-enthusiatic: its possible
for an XS sub to save PL_op and change its value. The old value will
only get restored when pp_entersub soes LEAVE, which is *after* we
cacluate gimme. So grab the value before the XS sub is called.