Rename imports nomenclature in path_range_query to exit_dependencies.

Rename imports nomenclature in path_range_query to exit_dependencies.

The purpose of this change is to disambiguate the imports name with
its use in GORI.

gcc/ChangeLog:

	* gimple-range-path.cc (path_range_query::import_p): Rename to...
	(path_range_query::exit_dependency_p): ...this.
	(path_range_query::dump): Rename imports to exit dependencies.
	(path_range_query::compute_ranges_in_phis): Same.
	(path_range_query::compute_ranges_in_block): Same.
	(path_range_query::adjust_for_non_null_uses): Same.
	(path_range_query::compute_ranges): Same.
	(path_range_query::compute_phi_relations): Same.
	(path_range_query::add_to_imports): Rename to...
	(path_range_query::add_to_exit_dependencies): ...this.
	(path_range_query::compute_imports): Rename to...
	(path_range_query::compute_exit_dependencies): ...this.
	* gimple-range-path.h (class path_range_query): Rename imports to
	exit dependencies.