transformExpr() did the Wrong Thing if applied to a SubLink node that

had already been transformed.  This led to failure in examples like
UPDATE table SET fld = (SELECT ...).  Repair this, and revise the
comments to explain that transformExpr has to be robust against this
condition.  Someday we might want to fix the callers so that
transformExpr is never invoked on its own output, but that someday
is not today.