leave_scope(): pop args in each branch

leave_scope(): pop args in each branch

About 4 years ago I heavily refactored Perl_leave_scope(). One of the
things I did was to pop the args for each action once at the top of the
loop. This involved a number of nested condtionals.

This commit makes each action (i.e. each switch branch) responsible
for copying its own args off the save stack, while the top of loop is
still responsible for decrementing PL_savestack_ix.

The gain is marginal, but it makes the code cleaner and the object code
smaller.