Refactor initial hash lookup in dynahash.c

Refactor initial hash lookup in dynahash.c

The same pattern is used three times in dynahash.c to retrieve a bucket
number and a hash bucket from a hash value.  This has popped up while
discussing improvements for the type cache, where this piece of
refactoring would become useful.

Note that hash_search_with_hash_value() does not need the bucket number,
just the hash bucket.

Author: Teodor Sigaev
Reviewed-by: Aleksander Alekseev, Michael Paquier
Discussion: https://postgr.es/m/5812a6e5-68ae-4d84-9d85-b443176966a1@sigaev.ru