In the MULTI-INDEX OR query plan, code for sub-expressions can sometimes be

generated twice.  But for some subqueries, generating code off of the same
tree twice causes problems.  So now MULTI-INDEX OR makes a copy of the
sub-expressions it uses to avoid code-generating them more than once.
dbsqlfuzz 9ebd2140e7206ff724e665f172faea28af801635.

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