Order of operations in lazy_vacuum_rel

I see that lazy_vacuum_rel() truncates the heap before it does vacuuming
of the free space map.  Once upon a time this wouldn't have mattered,
but now it means that cancel interrupts are likely to be ignored for
the duration of FreeSpaceMapVacuum().  Is that really necessary?
Would it be okay to swap the two steps?
        regards, tom lane