pgsql: Tweak order of operations in BitmapHeapNext() to avoid the case

Log Message:
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Tweak order of operations in BitmapHeapNext() to avoid the case of prefetching
the same page we are nanoseconds away from reading for real.  There should be
something left to do on the current page before we consider issuing a prefetch.

Modified Files:
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    pgsql/src/backend/executor:
        nodeBitmapHeapscan.c (r1.33 -> r1.34)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeBitmapHeapscan.c?r1=1.33&r2=1.34)