SpecConstr: accommodate casts in value arguments

This commit:

  commit fb050a33
  Author: Simon Peyton Jones <simonpj@microsoft.com>
  Date:   Thu Oct 12 11:00:19 2017 +0100

  Do not bind coercion variables in SpecConstr rules

arranged to reject any SpecConstr call pattern that mentioned
a coercion in the pattern.

There was a good reason for that
-- see Note [SpecConstr and casts] --
but I didn't realise how important it was to accept patterns
that mention casts in /terms/.  Trac #14936 showed this up.

This patch just narrows the restriction to discard only
the cases where the coercion is mentioned only in types.
Fortunately that was pretty easy to do.