pgsql: BitmapHeapScan: begin scan after bitmap creation

BitmapHeapScan: begin scan after bitmap creation

Change the order so that the table scan is initialized only after
initializing the index scan and building the bitmap.

This is mostly a cosmetic change for now, but later commits will need
to pass parameters to table_beginscan_bm() that are unavailable in
ExecInitBitmapHeapScan().

Author: Melanie Plageman
Reviewed-by: Tomas Vondra, Andres Freund, Heikki Linnakangas
Discussion: https://postgr.es/m/CAAKRu_ZwCwWFeL_H3ia26bP2e7HiKLWt0ZmGXPVwPO6uXq0vaA%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/1577081e9614345534a018e788a2c0bab4da4dc5

Modified Files
--------------
src/backend/executor/nodeBitmapHeapscan.c | 27 ++++++++++++++++++++-------
1 file changed, 20 insertions(+), 7 deletions(-)