pgsql: Suppress creation of backwardly-indexed paths for LATERAL join c

Suppress creation of backwardly-indexed paths for LATERAL join clauses.

Given a query such as

SELECT * FROM foo JOIN LATERAL (SELECT foo.var1) ss(x) ON ss.x = foo.var2

the existence of the join clause "ss.x = foo.var2" encourages indxpath.c to
build a parameterized path for foo using any index available for foo.var2.
This is completely useless activity, though, since foo has got to be on the
outside not the inside of any nestloop join with ss. It's reasonably
inexpensive to add tests that prevent creation of such paths, so let's do
that.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/77387f0ac859c099c6ab669db1e7a852524696c4

Modified Files
--------------
src/backend/optimizer/path/equivclass.c | 10 +++++++-
src/backend/optimizer/path/indxpath.c | 41 ++++++++++++++++++++++++++----
src/include/optimizer/paths.h | 3 +-
3 files changed, 46 insertions(+), 8 deletions(-)