Replace the switching function ExecEvalExpr() with a macro that jumps
directly to the appropriate per-node execution function, using a function pointer stored by ExecInitExpr. This speeds things up by eliminating one level of function call. The function-pointer technique also enables further small improvements such as only making one-time tests once (and then changing the function pointer). Overall this seems to gain about 10% on evaluation of simple expressions, which isn't earthshaking but seems a worthwhile gain for a relatively small hack. Per recent discussion on pghackers.
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