Fix a buglet in Simplify.simplCast

This bug, revealed by #18347, is just a missing update to
sc_hole_ty in simplCast.  I'd missed a code path when I
made the recentchanges in

    commit 6d49d5be
    Author: Simon Peyton Jones <simonpj@microsoft.com>
    Date:   Thu May 21 12:53:35 2020 +0100

    Implement cast worker/wrapper properly

The fix is very easy.

Two other minor changes

* Tidy up in SimpleOpt.simple_opt_expr. In fact I think this is an
  outright bug, introduced in the fix to #18112: we were simplifying
  the same coercion twice *with the same substitution*, which is just
  wrong.  It'd be a hard bug to trigger, so I just fixed it; less code
  too.

* Better debug printing of ApplyToVal