analyzer: fix sense in range::add_bound [PR94362]

analyzer: fix sense in range::add_bound [PR94362]

Mikael Morin spotted that I got the sense wrong when discarding
redundant constraints in
r12-6782-gc4b8f3730a80025192fdb485ad2535c165340e41.

Fixed as follows, which also moves the rejection of contradictory
constraints in range::add_bound to earlier, so that this code can
be self-tested.

gcc/analyzer/ChangeLog:
	PR analyzer/94362
	* constraint-manager.cc (range::add_bound): Fix tests for
	discarding redundant constraints.  Perform test for rejecting
	unsatisfiable constraints earlier so that they don't update
	the object on failure.
	(selftest::test_range): New.
	(selftest::test_constant_comparisons): Add test coverage for
	existing constraints becoming narrower until they are
	unsatisfiable.
	(selftest::run_constraint_manager_tests): Call test_range.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>