fix prompt in fix in single-value context compilation

fix prompt in fix in single-value context compilation

* module/language/tree-il/compile-glil.scm (flatten): When compiling a
  <prompt> in push context with an RA, after the body returns normally,
  jump to that RA instead of to our POST label (which in that case does
  not need to be emitted).  Fixes a tail <prompt> in a push <fix>.

* test-suite/tests/control.test ("prompt in different contexts"): Add
  more test cases.