toke.c: Allocate __SUB__ as a baseop if possible

__SUB__ is a baseop, but it gets converted to a const op (a svop)
later if the sub is not clonable.  Since a svop is bigger, we allo-
cate it as a larger size initially.  But if we already know that
the sub is clonable by the time we see __SUB__, we can go with the
smaller op size.
Deparse +sub :attr{} correctly at stmt start