Add 'undo_ptr_list_last()' helper function

Add 'undo_ptr_list_last()' helper function

It's like 'delete_ptr_list_last()', except it doesn't de-allocate the
ptrlist block that it removes the last entry from.  Useful when you know
you're going to add things back (ie basic block tail rewriting), and
when you can't afford to de-allocate the ptrlist because you're in a
loop.