Suppose we have
        type family T a :: * -> *
        type instance T Int = []

and now we encounter the type (T Int Bool).  That is perfectly
fine, even though T is over-saturated here.

This patch makes lookupFamInstEnv robust to such over-saturation.
Previously one caller (TcTyFuns.tcUnfoldSynFamInst) dealt with
the over-saturation case, but the others did not. It's better
to desl with the issue at the root, in lookupFamInstEnv itself.