Piecewise: input should work whether its tuple or ExprCondPair

Should fix `S(srepr(p)) == p` where `p` is a Piecewise.

If the Piecewise() gets an existing ExprCondPair it should not try
to deference it like a Tuple to build a new ExprCondPair.  Rather
it should just use the ExprCondPair.

But help!  It doesn't work...
Sorry @skirpichev if I was unclear: my commit shows "isinstance" not working.  But I also tried `type(ec) == ExprCondPair` and anything else I could think of :)