Update string_constant for new const initializer code

[Jan Hubicka]

> 
> So if it is replaceable then why is the check necessary?  The function
> looks at DECL_INITIAL anyway and I hope that is not set when
> !varpool_get_node (array)->const_value_known but DECL_REPLACEABLE_P is
> true?
> 
> I'd hate to have two mechanisms that can easily get out of sync.
> 
> But maybe I'm missing sth.

The problem is opposite.  You have DECL_REPLACEABLE_P false, but still
varpool_get_node (array)->const_value_known is true because we remember from
WPA stage that the initializer will be put into other ltrans partition.

Honza
> 
> Richard.