### 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
assertFalse(r.includeUpper());
    }
    
    public void testCombineMixedMultipleBinaryComparisons() {
        FieldAttribute fa = getFieldAttribute();
        GreaterThanOrEqual gte = greaterThanOrEqualOf(fa, THREE);
        GreaterThan gt = greaterThanOf(fa, FOUR);
        LessThanOrEqual lte = lessThanOrEqualOf(fa, L(7));
        Expression ne = new Not(EMPTY, equalsOf(fa, FIVE));
        CombineBinaryComparisons rule = new CombineBinaryComparisons();
        
        Expression exp = rule.rule(new And(EMPTY, gte, new And(EMPTY, TRUE, new And(EMPTY, gt, new And(EMPTY, ne, lte)))));
        assertEquals(And.class, exp.getClass());
        And and = ((And) exp);
        assertEquals(Range.class, and.right().getClass());
        Range r = (Range) and.right();
        assertEquals(FOUR, r.lower());
        assertFalse(r.includeLower());
        assertEquals(L(7), r.upper());
        assertTrue(r.includeUpper());
    }
    
    public void testCombineComparisonsIntoRange() {
        FieldAttribute fa = getFieldAttribute();
        GreaterThanOrEqual gte = greaterThanOrEqualOf(fa, ONE);
        LessThan lt = lessThanOf(fa, FIVE);
        CombineBinaryComparisons rule = new CombineBinaryComparisons();
        Expression exp = rule.rule(new And(EMPTY, gte, lt));
        assertEquals(Range.class, exp.getClass());
        Range r = (Range) exp;