isl_basic_map_project_out: add check for existentially quantified variables
isl_basic_map_project_out: add check for existentially quantified variables

It does not make sense to try and project out variables that have
already been projected out.  The implementation probably does not
handle this case very gracefully, but there is also no point in
trying to do so.  Simply make clear that this is not allowed.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

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