Substitution should just substitute, not optimise
This was causing Trac #4524, by optimising (e |> co) to e on the LHS of a rule. Result, the template variable 'co' wasn't bound any more. Now that substition doesn't optimise, it seems sensible to call simpleOptExpr rather than substExpr when substituting in the RHS of rules. Not a big deal either way.
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