Minor cleanups in the BRIN code

Minor cleanups in the BRIN code

BRIN bloom and minmax-multi opclasses were somewhat inconsistent when
dealing with bool variables, assigning to them Datum values etc. While
not a bug, it makes the code harder to understand, so fix that.

While at it, update an incorrect comment copied to bloom opclass from
minmax, talking about strategies not supported by bloom.

Reviewed-by: Heikki Linnakangas
Discussion: https://postgr.es/m/0e1f3350-c9cf-ab62-43a5-5dae314de89c%40enterprisedb.com