### 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
CombineBinaryComparisons rule = new CombineBinaryComparisons();
        Expression exp = rule.rule(or);
        assertEquals(Or.class, exp.getClass());
        Or ro = (Or) exp;
        assertEquals(LessThan.class, ro.left().getClass());
        LessThan lt = (LessThan) ro.left();
        assertEquals(TWO, lt.right());
        assertEquals(GreaterThan.class, ro.right().getClass());
        GreaterThan gt = (GreaterThan) ro.right();
        assertEquals(THREE, gt.right());
    }
    
    public void testCombineBinaryComparisonsDisjunctionOfIncludedRangeNotComparable() {
        FieldAttribute fa = getFieldAttribute();
        Range r1 = rangeOf(fa, TWO, false, THREE, false);
        Range r2 = rangeOf(fa, ONE, false, FALSE, false);
        Or or = new Or(EMPTY, r1, r2);
        CombineBinaryComparisons rule = new CombineBinaryComparisons();
        Expression exp = rule.rule(or);
        assertEquals(or, exp);
    }
    
    public void testCombineBinaryComparisonsDisjunctionOfIncludedRange() {
        FieldAttribute fa = getFieldAttribute();
        Range r1 = rangeOf(fa, TWO, false, THREE, false);
        Range r2 = rangeOf(fa, ONE, false, FOUR, false);
        Or or = new Or(EMPTY, r1, r2);
        CombineBinaryComparisons rule = new CombineBinaryComparisons();
        Expression exp = rule.rule(or);
        assertEquals(Range.class, exp.getClass());