BRIN Scan: Optimize memory allocation in function 'bringetbitmap'

BRIN Scan: Optimize memory allocation in function 'bringetbitmap'.
We can allocate memory for some pointer before do long loop instead of allocating
memory in long loop.

Before optimizing code (warm run)
postgres=# select count(*) from lineitem where l_orderkey=1;
count
-------
6
(1 row)

Time: 456.219 ms

After optimizing code (warm run)
postgres=# select count(*) from lineitem where l_orderkey=1;
count
-------
6
(1 row)

Time: 349.219 ms

The following shows the DDL of this test case.
CREATE TABLE LINEITEM ( L_ORDERKEY INTEGER NOT NULL,
L_PARTKEY INTEGER NOT NULL,
L_SUPPKEY INTEGER NOT NULL,
L_LINENUMBER INTEGER NOT NULL,
L_QUANTITY DECIMAL(15,2) NOT NULL,
L_EXTENDEDPRICE DECIMAL(15,2) NOT NULL,
L_DISCOUNT DECIMAL(15,2) NOT NULL,
L_TAX DECIMAL(15,2) NOT NULL,
L_RETURNFLAG CHAR(1) NOT NULL,
L_LINESTATUS CHAR(1) NOT NULL,
L_SHIPDATE DATE NOT NULL,
L_COMMITDATE DATE NOT NULL,
L_RECEIPTDATE DATE NOT NULL,
L_SHIPINSTRUCT CHAR(25) NOT NULL,
L_SHIPMODE CHAR(10) NOT NULL,
L_COMMENT VARCHAR(44) NOT NULL);

copy lineitem from '/home/jinyu/mywork/dbgen/lineitem.tbl' delimiter '|';
create index brinLineitem on lineitem using brin(L_ORDERKEY) with(pages_per_range = 1);

Jinyu Zhang