pgsql: Fix performance issue in exprTypmod(): for a COALESCE expression,

Log Message:
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Fix performance issue in exprTypmod(): for a COALESCE expression, it
recursed twice on its first argument, leading to exponential time spent
on a deep nest of COALESCEs ... such as a deeply nested FULL JOIN would
produce.  Per report from Matt Carter.

Tags:
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REL8_0_STABLE

Modified Files:
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    pgsql/src/backend/parser:
        parse_expr.c (r1.179.4.2 -> r1.179.4.3)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_expr.c.diff?r1=1.179.4.2&r2=1.179.4.3)