Better fix for RT #2140 (list assignment with duplicated temporaries)

Better fix for RT #2140 (list assignment with duplicated temporaries)

4c8f17b905f2 (change 7867) took the approach of a special case in sv_setsv()
when PL_op indicated that the current OP was OP_AASSIGN. The problem is in one
part of pp_aassign, where it was using sv_mortalcopy() on values that were
correctly marked as temporaries, but also still needed later. Hence a more
targetted solution is to avoid that call, and to instead use API calls that
will not steal temporaries.