isl_ast_codegen.c: compute_class_domains: compute gist of class domains

isl_ast_codegen.c: compute_class_domains: compute gist of class domains

The class domains may reference variables that been eliminated from
the "executed" map.  Constraints involving these variables may
therefore get accidentally reintroduced into the "executed" map,
which can render the AST build unable to detect whether the inner
"executed" map is single-valued, possibly causing an infinite recursion.
Plug in the values that caused the variables to be eliminated from
the "executed" map to remove any reference to them from the class domains.

Reported-by: Tobias Grosser <tobias@grosser.es>
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>