pgsql: Grab some low-hanging fruit in the new hash index build code.

Log Message:
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Grab some low-hanging fruit in the new hash index build code.
oprofile shows that a nontrivial amount of time is being spent in
repeated calls to index_getprocinfo, which really only needs to be
called once.  So do that, and inline _hash_datum2hashkey to make it
work.

Modified Files:
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    pgsql/src/backend/utils/sort:
        tuplesort.c (r1.82 -> r1.83)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/sort/tuplesort.c?r1=1.82&r2=1.83)