ast/compile: reorder body for safety differently (#4801)

Our previous approach, ordering for closures first, and taking the growing
set of output variables of the reordered body into account in a second step,
did not work out for some examples:

    object.get(input.subject.roles[_], comp, [""], output)
    comp = [ 1 | true ]
    every y in [2] {
    	y in output
    }

Here, the closure of `every` would have not checkout out because we've never
registered the output variable `output` -- since the first call to object.get
is unsafe without `comp`, too.

Now, the two stages have been merged. It's got surprisingly little fallout,
one test case had to be adjusted, which I believe to be a minor case, too.

Fixes the second part of #4766.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>