Fix wrong field order in _readMergeWhenClause().

We hadn't noticed this because it's dead code: there is no
situation where we read raw parse trees from text format.
So maybe the right fix is to remove the function altogether,
but I'll forbear for now; it's not the only dead code in
readfuncs.c, I think.

Noted while comparing existing code to the results of
Peter's auto-generation script.

src/backend/nodes/readfuncs.c

@@ -485,10 +485,10 @@ _readMergeWhenClause(void)
 
 	READ_BOOL_FIELD(matched);
 	READ_ENUM_FIELD(commandType, CmdType);
+	READ_ENUM_FIELD(override, OverridingKind);
 	READ_NODE_FIELD(condition);
 	READ_NODE_FIELD(targetList);
 	READ_NODE_FIELD(values);
-	READ_ENUM_FIELD(override, OverridingKind);
 
 	READ_DONE();
 }