unordered_multimap/unordered_multiset optimizations

[François Dumont]

Hi

 Â Â Â  Still influenced by PR 68303 this patch:

- Extend usage of find within other methods. It simplify code and will 
allow to implement the PR in less places if we decide to do so.

- Get rid of several bucket index comparison for non-unique key 
containers this way we have less hash code computations.

It also adds _M_update_bbegin cause I plan to propose another 
optimization regarding this special bucket maintenance.


 Â Â Â  * include/bits/hashtable_policy.h (_Map_base<>::at): Use
 Â Â Â  _Hashtable<>::find.
(_Hashtable_base<>::_Equal_hash_code<>::_S_node_equals):New.
 Â Â Â  (_Hashtable_base<>::_M_node_equals): New, use latter.
 Â Â Â  * include/bits/hashtable.h (_Hashtable<>::_M_update_bbegin): New.
 Â Â Â  (_Hashtable<>::_M_assign): Use latter.
 Â Â Â  (_Hashtable<>::_M_move_assign): Likewise.
 Â Â Â  (_Hashtable<>(_Hashtable<>&&)): Likewise.
 Â Â Â  (_Hashtable<>(_Hashtable<>&&, const allocator_type&)): Likewise.
 Â Â Â  (_Hashtable<>::swap): Likewise.
 Â Â Â  (_Hashtable<>::find): Build iterator directly from _M_find_node result.
 Â Â Â  (_Hashtable<>::count): Use _Hashtable<>::find.
 Â Â Â  (_Hashtable<>::equal_range): Likewise.
 Â Â Â  (_Hashtable<>::_M_erase(false_type, const key_type&)): Use
 Â Â Â  _M_node_equals.

Tested under Linux x86_64, ok to commit ?

François

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