Fix a bunch of bad interactions between partial indexes and the new

Fix a bunch of bad interactions between partial indexes and the new
planning logic for bitmap indexscans.  Partial indexes create corner
cases in which a scan might be done with no explicit index qual conditions,
and the code wasn't handling those cases nicely.  Also be a little
tenser about eliminating redundant clauses in the generated plan.
Per report from Dmitry Karasik.