Fix oversight in recent parameterized-path patch.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 7c85aa39fc08df44e1ce67e651bda4cf7e331580
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2012-04-26T18:17:44Z
Releases: 9.2.0
Fix oversight in recent parameterized-path patch.

bitmap_scan_cost_est() has to be able to cope with a BitmapOrPath, but
I'd taken a shortcut that didn't work for that case.  Noted by Heikki.
Add some regression tests since this area is evidently under-covered.

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