avoid generation of empty guards in trivial cases

We used to insert a trivial integer_onep condition around loops we know have
more than an iteration.  This patch avoids generating those conditions.

	* graphite-isl-ast-to-gimple.c (graphite_create_new_loop_guard): Do not call
	create_empty_if_region_on_edge when cond_expr is true.
	(translate_isl_ast_node_for): Check whether a guard has been generated.

Co-Authored-By: Sebastian Pop <s.pop@samsung.com>

From-SVN: r229137