Small improvement to get_base_rel_indexes()

Attached is a small patch which improves the way get_base_rel_indexes() works.

The current version creates a new bitmapset on each recursion level then bms_joins() to the one on the next level up each time. I understand that this will patch will have about a 0 net performance improvement, but I thought I'd post anyway as:

1. It removes 5 lines of code.

2. It's a better example to leave in the code.

Is it worth applying?

Regards

David Rowley