The simplifier is taking more iterations than it should, because we
were fruitlessly ANF-ing a top-level declaration of form

   x = Ptr "foo"#

to get
 
   x = let v = "foo"# in Ptr v

and then inlining v again.  This patch makes Simplify.makeTrivial 
top-level aware, so that it doesn't ANF if it's going to be undone.