pgsql: Fix oversight in 8.0 modification of RestrictInfo data

Log Message:
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Fix oversight in 8.0 modification of RestrictInfo data structures.
A RestrictInfo representing an OR clause now contains two versions of
the contained expression, one with sub-RestrictInfos and one without.
clause_selectivity() should descend to the version with sub-RestrictInfos
so that it has a chance of caching its results for the OR's sub-clauses.
Failing to do so resulted in redundant planner effort.

Tags:
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REL8_0_STABLE

Modified Files:
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    pgsql/src/backend/optimizer/path:
        clausesel.c (r1.72 -> r1.72.4.1)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/clausesel.c.diff?r1=1.72&r2=1.72.4.1)