fix nontail loops within loops

fix nontail loops within loops

* module/language/tree-il/compile-glil.scm (flatten): Fix compilation of
  loops within loops in non-tail positions. Will add a test case soon,
  but one way to reproduce it was with the following function:

(define (test)
  (let lp ()
    (pk 'zero)
    (let ((fk (lambda ()
                (let ((fk2 (lambda () (pk 'two))))
                  (let ((fk3 (lambda () (if #t (pk 'three) (fk2)))))
                    (if #t
                        (fk3)
                        (fk2)))))))
      (pk 'one)
      (fk))
    (lp)))

One would expect to see a sequence of "zero one three", but in fact zero
only showed once.

This should fix simplex as well.