Fix Trac #7453 on the 7.6 branch

This patch fixes a quite egregious bug. runTcS was intialising
the 'untouchables' to NoUntouchables, which is quite wrong for
the invocation in TcSimplify.simplifyInfer.  Result: missed
skolem-escape check and seg-fault city.

All this is done differently, and better, in HEAD, so this
patch is needed only on the branch.