isl_pw_*_gist: avoid intersection of domain with context
isl_pw_*_gist: avoid intersection of domain with context

The intersection was introduced in cf42ba2 (isl_pw_qpolynomial_gist:
substitute equalities in polynomials, Mon Oct 25 13:49:22 2010 +0200)
in order to be able to detect and exploit equalities in the intersection.
However, there is no need to change the domain itself as we can just
as well use a temporary object.

Intersecting the domain with the context is not what the user would
expect as the purpose of the gist operation is to only keep the most
salient information, which is quite the opposite of introducing
extra constraints.

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

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