Merge: Filter isa adaptation

`isa` used to always adapt a local variable, even if there was strict information loss (a warning was shown but the adaptation was effective).

This caused issues for the adaptive typing system since there was no way to retrieve the original type information.

~~~nit
var a: A
a = new B
# a is now a B
if a isa A then
   # warning, but a is now a A
else
  # nothing here, so a is still a B
end
# merge type information of both branches: a isa A or B, thus a isa A
~~~

The change is now that `a isa A` still cause a warning but not more do the adaptation.
Thus in the *then* branch, `a` is still a `B`.
Thus in the merge at the end of the `if`, `a` remains a `B`.

close #2202

Pull-Request: #2203
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>