### Description

The IS NULL/IS NOT NULL only care about the nullability not the value of an expression.
Given the expression x + 1 / 2, the actual result does not matter only if it's null or not - that is, it only matters if x is null or not.
So x + 1 / 2 IS NULL becomes x IS NULL - which can be opportunistically pushed down or evaluated.
x-pack/plugin/ql/src/test/java/org/elasticsearch/xpack/ql/optimizer/OptimizerRulesTests.java
FieldAttribute fa = getFieldAttribute();
        NotEquals neq = notEqualsOf(fa, TWO);
        LessThanOrEqual lte = lessThanOrEqualOf(fa, TWO);
        And and = new And(EMPTY, neq, lte);
        CombineBinaryComparisons rule = new CombineBinaryComparisons();
        Expression exp = rule.rule(and);
        assertEquals(LessThan.class, exp.getClass());
        LessThan lt = (LessThan) exp;
        assertEquals(TWO, lt.right());
    }
    
    public void testCombineBinaryComparisonsConjunction_Neq2AndLte1() {
        FieldAttribute fa = getFieldAttribute();
        NotEquals neq = notEqualsOf(fa, TWO);
        LessThanOrEqual lte = lessThanOrEqualOf(fa, ONE);
        And and = new And(EMPTY, neq, lte);
        CombineBinaryComparisons rule = new CombineBinaryComparisons();
        Expression exp = rule.rule(and);
        assertEquals(lte, exp);
    }
    
    public void testCombineBinaryComparisonsDisjunctionNotComparable() {
        FieldAttribute fa = getFieldAttribute();
        GreaterThan gt1 = greaterThanOf(fa, ONE);
        GreaterThan gt2 = greaterThanOf(fa, FALSE);
        Or or = new Or(EMPTY, gt1, gt2);
        CombineBinaryComparisons rule = new CombineBinaryComparisons();
        Expression exp = rule.rule(or);
        assertEquals(exp, or);
    }