pgsql: Refactor transformExpr() by creating separate functions for most

Log Message:
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Refactor transformExpr() by creating separate functions for most of the
expression types.

Modified Files:
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pgsql/src/backend/parser:
parse_expr.c (r1.179 -> r1.180)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_expr.c.diff?r1=1.179&r2=1.180)