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

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.