Core allows non-recursive type-lets, thus

   let a = TYPE ty in ...

They are substituted away very quickly, but it's convenient for
some passes to produce them (rather than to have to substitute
immediately).

Trac #13708 tried the effect of not running the simplifer at all
(a rather bizarre thing to do, but still).  That showed that some
passes crashed because they always treated a let-bounder binder
as an Id.  This patch adds some easy fixes.