exprTableUsage
mask = exprTableUsage(pMaskSet, p->pRight);
mask |= exprTableUsage(pMaskSet, p->pLeft);
mask |= exprTableUsage(pMaskSet, p->pList->a[i].pExpr);
pInfo->prereqLeft = exprTableUsage(pMaskSet, pExpr->pLeft);
pInfo->prereqRight = exprTableUsage(pMaskSet, pExpr->pRight);
pInfo->prereqAll = exprTableUsage(pMaskSet, pExpr);