pgsql: Marginal performance hack: remove the loop that used to be needed

Log Message:
-----------
Marginal performance hack: remove the loop that used to be needed to
look through a freelist for a chunk of adequate size. For a long time
now, all elements of a given freelist have been exactly the same
allocated size, so we don't need a loop. Since the loop never iterated
more than once, you'd think this wouldn't matter much, but it makes a
noticeable savings in a simple test --- perhaps because the compiler
isn't optimizing on a mistaken assumption that the loop would repeat.
AllocSetAlloc is called often enough that saving even a couple of
instructions is worthwhile.

Modified Files:
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pgsql/src/backend/utils/mmgr:
aset.c (r1.71 -> r1.72)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mmgr/aset.c.diff?r1=1.71&r2=1.72)