Fix possible crash with Parallel Bitmap Heap Scan.
If a Parallel Bitmap Heap scan's chain of leftmost descendents
includes a BitmapOr whose first child is a BitmapAnd, the prior coding
would mistakenly create a non-shared TIDBitmap and then try to perform
shared iteration.
Report by Tomas Vondra. Patch by Dilip Kumar.
Discussion: http://postgr.es/m/
50e89684-8ad9-dead-8767-
c9545bafd3b6@2ndquadrant.com