Make TmOracle reduce nullary constructor equalities

Previously, `simplifyEqExpr` would just give up on constructor
applications when no argument to either constructor was simplified.

That's unfortunate as all arguments might be equal anyway! A special
case of this is nullary constructors. Currently, the TmOracle would fail
to solve a term equality `False ~ (True = True)` because it can't make
any progress on any of `True`s arguments.

Instead we now try to properly simplify the term equality even when no
simplification of constructor arguments was achieved.

compiler/deSugar/TmOracle.hs

@@ -210,7 +210,7 @@ simplifyEqExpr e1 e2 = case (e1, e2) of
             worst_case  = PmExprEq (PmExprCon c1 ts1') (PmExprCon c2 ts2')
         in  if | not (or bs1 || or bs2) -> (worst_case, False) -- no progress
                | all isTruePmExpr  tss  -> (truePmExpr, True)
-               | any isFalsePmExpr tss  -> (falsePmExpr, True)
+               | otherwise              -> (worst_case, or bs1 || or bs2)
                | otherwise              -> (worst_case, False)
     | otherwise -> (falsePmExpr, True)