The problem with tagToEnum# is that it is not overloaded (in the
Haskell sense) but you are only supposed to apply it to a TyCon
that is an enumeration (isEnumerationTyCon).

The Real Way to do this is to have some special kind of type constraint
for the purpose, but that is wild overkill. So this patch adds a small
rather ad-hoc check to TcExpr.instFun.  Crude, simple, but it works fine.

Fixes Trac #786
Test is tcfail164