### 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 testCombineBinaryComparisonsAndRangeUpperEqual() {
        FieldAttribute fa = getFieldAttribute();
        LessThanOrEqual lte = lessThanOrEqualOf(fa, TWO);
        Range range = rangeOf(fa, ONE, false, THREE, false);
        CombineBinaryComparisons rule = new CombineBinaryComparisons();
        Expression exp = rule.rule(new And(EMPTY, lte, range));
        assertEquals(Range.class, exp.getClass());
        Range r = (Range) exp;
        assertEquals(ONE, r.lower());
        assertFalse(r.includeLower());
        assertEquals(TWO, r.upper());
        assertTrue(r.includeUpper());
    }
    
    public void testCombineMultipleBinaryComparisons() {
        FieldAttribute fa = getFieldAttribute();
        GreaterThanOrEqual gte = greaterThanOrEqualOf(fa, THREE);
        GreaterThan gt = greaterThanOf(fa, FOUR);
        LessThanOrEqual lte = lessThanOrEqualOf(fa, L(7));
        LessThan lt = lessThanOf(fa, SIX);
        CombineBinaryComparisons rule = new CombineBinaryComparisons();
        Expression exp = rule.rule(new And(EMPTY, gte, new And(EMPTY, gt, new And(EMPTY, lt, lte))));
        assertEquals(Range.class, exp.getClass());
        Range r = (Range) exp;
        assertEquals(FOUR, r.lower());
        assertFalse(r.includeLower());
        assertEquals(SIX, r.upper());