Do not factor out constant functions into the initialization section of a

Do not factor out constant functions into the initialization section of a
prepared statement, because even though they are constant, they can still
throw exceptions.  Instead, put such functions in an OP_Once block.  This
fixes ticket [3c9eadd2a6ba0aa5] and causes COALESCE() and CASE...END to
be short-circuit.

FossilOrigin-Name: c5f96a085db9688a09793f52ce1ecf033c2e6e2e5873a19fe0fb374b242b317f