hash-table.h: support non-zero empty values in empty_slow (v2)

hash-table.h: support non-zero empty values in empty_slow (v2)

gcc/cp/ChangeLog:
	* cp-gimplify.c (source_location_table_entry_hash::empty_zero_p):
	New static constant.
	* cp-tree.h (named_decl_hash::empty_zero_p): Likewise.
	(struct named_label_hash::empty_zero_p): Likewise.
	* decl2.c (mangled_decl_hash::empty_zero_p): Likewise.

gcc/ChangeLog:
	* attribs.c (excl_hash_traits::empty_zero_p): New static constant.
	* gcov.c (function_start_pair_hash::empty_zero_p): Likewise.
	* graphite.c (struct sese_scev_hash::empty_zero_p): Likewise.
	* hash-map-tests.c (selftest::test_nonzero_empty_key): New selftest.
	(selftest::hash_map_tests_c_tests): Call it.
	* hash-map-traits.h (simple_hashmap_traits::empty_zero_p):
	New static constant, using the value of = H::empty_zero_p.
	(unbounded_hashmap_traits::empty_zero_p): Likewise, using the value
	from default_hash_traits <Value>.
	* hash-map.h (hash_map::empty_zero_p): Likewise, using the value
	from Traits.
	* hash-set-tests.c (value_hash_traits::empty_zero_p): Likewise.
	* hash-table.h (hash_table::alloc_entries): Guard the loop of
	calls to mark_empty with !Descriptor::empty_zero_p.
	(hash_table::empty_slow): Conditionalize the memset call with a
	check that Descriptor::empty_zero_p; otherwise, loop through the
	entries calling mark_empty on them.
	* hash-traits.h (int_hash::empty_zero_p): New static constant.
	(pointer_hash::empty_zero_p): Likewise.
	(pair_hash::empty_zero_p): Likewise.
	* ipa-devirt.c (default_hash_traits <type_pair>::empty_zero_p):
	Likewise.
	* ipa-prop.c (ipa_bit_ggc_hash_traits::empty_zero_p): Likewise.
	(ipa_vr_ggc_hash_traits::empty_zero_p): Likewise.
	* profile.c (location_triplet_hash::empty_zero_p): Likewise.
	* sanopt.c (sanopt_tree_triplet_hash::empty_zero_p): Likewise.
	(sanopt_tree_couple_hash::empty_zero_p): Likewise.
	* tree-hasher.h (int_tree_hasher::empty_zero_p): Likewise.
	* tree-ssa-sccvn.c (vn_ssa_aux_hasher::empty_zero_p): Likewise.
	* tree-vect-slp.c (bst_traits::empty_zero_p): Likewise.
	* tree-vectorizer.h
	(default_hash_traits<scalar_cond_masked_key>::empty_zero_p):
	Likewise.