FIX : teach expression walker about RestrictInfo

Hi there,

the attached trivial patch adds handling of RestrictInfo nodes into
expression_tree_walker(). This is needed for example when calling
pull_varnos or (or other functions using the expression walker) in
clausesel.c, for example. An example of a query causing errors with
pull_varnos is

select * from t where (a >= 10 and a <= 20) or (b >= 15 and b <= 20);

which gets translated into an expression tree like this:

BoolExpr [OR_EXPR]
BoolExpr [AND_EXPR]
RestrictInfo
OpExpr [Var >= Const]
RestrictInfo
OpExpr [Var <= Const]
BoolExpr [AND_EXPR]
RestrictInfo
OpExpr [Var >= Const]
RestrictInfo
OpExpr [Var <= Const]

and the nested RestrictInfo nodes make the walker fail because of
unrecognized node.

It's possible that expression walker is not supposed to know about
RestrictInfo, but I don't really see why would that be the case.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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