libstdc++: Review _Hashtable count, equal_range _M_erase(false_type,) code

libstdc++: Review _Hashtable count, equal_range _M_erase(false_type,) code

Simplify operator[] implementation using find method. Review several
_Hashtable method implementations to limit the computation of bucket index.
Introduce _M_update_bbegin to simplify code.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable_policy.h (_Map_base<>::at): Use
	_Hashtable<>::find.
	(_Hashtable_base<>::_Equal_hash_code<>::_S_node_equals):New.
	(_Hashtable_base<>::_M_node_equals): New, use latter.
	(_Equality<_Key, _Value, _Alloc, _ExtractKey, _Equal, _H1, _H2, _Hash,
	_RehashPolicy, false>::_M_equal): Adapt to use latter.
	* include/bits/hashtable.h (_Hashtable<>::_M_update_bbegin): New.
	(_Hashtable<>::_M_assign): Use latter.
	(_Hashtable<>::_M_move_assign): Likewise.
	(_Hashtable<>(_Hashtable<>&&)): Likewise.
	(_Hashtable<>(_Hashtable<>&&, const allocator_type&)): Likewise.
	(_Hashtable<>::swap): Likewise.
	(_Hashtable<>::find): Build iterator directly from _M_find_node result.
	(_Hashtable<>::count): Use _Hashtable<>::find.
	(_Hashtable<>::equal_range): Likewise.
	(_Hashtable<>::_M_erase(false_type, const key_type&)): Use
	_M_node_equals.