Fix wrong field order in _readMergeWhenClause().

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.