pgsql: Allow create_index_paths() to consider multiple join bitmapscan

Allow create_index_paths() to consider multiple join bitmapscan paths.

In the initial cut at the "parameterized paths" feature, I'd simplified
create_index_paths() to the point where it would only generate a single
parameterized bitmap path per relation. Experimentation with an example
supplied by Josh Berkus convinces me that that's not good enough: we really
need to consider a bitmap path for each possible outer relation. Otherwise
we have regressions relative to pre-9.2 versions, in which the planner
picks a plain indexscan where it should have used a bitmap scan in queries
involving three or more tables. Indeed, after fixing this, several queries
in the regression tests show improved plans as a result of using bitmap not
plain indexscans.

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/26893906896d25b0119c57c90474c153bb8083c9

Modified Files
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src/backend/optimizer/path/indxpath.c | 98 +++++++++++++++++++++++++++-----
src/test/regress/expected/join.out | 28 ++++++----
2 files changed, 99 insertions(+), 27 deletions(-)