performed. The resulting code is both more compact, and more general.

It is more  general, since that predef operators  are now represented
by  node_exp_eff, exactly as  user nodes.   Hence, all  the functions
that were  operating on  user nodes via  node_exp_eff (such  as, node
aliasing) works for free on predef op!

In order to be able  to perform that generalisation, it was necessary
to  extend sligthly  the data  structure used  to represent  the node
profile in CompiledData.node_exp_eff with information indicating if a
variable is polymorphic or overloaded.

Not that, currently, polymorphic  or overloaded variables can only be
introduced by predef  operators. But I think it would  be easy to add
those notions for normal user nodes after this change.

New non-reg files boolred now compiles. Those involving
- boolred
- alias on predef.op