### 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();
        And and = new And(EMPTY, equalsOf(fa, ONE), equalsOf(fa, TWO));
        Filter dummy = new Filter(EMPTY, relation(), and);
        LogicalPlan transformed = new CombineDisjunctionsToIn().apply(dummy);
        assertSame(dummy, transformed);
        assertEquals(and, ((Filter) transformed).condition());
    }
    public void testTwoEqualsDifferentFields() throws Exception {
        FieldAttribute fieldOne = getFieldAttribute("ONE");
        FieldAttribute fieldTwo = getFieldAttribute("TWO");
        Or or = new Or(EMPTY, equalsOf(fieldOne, ONE), equalsOf(fieldTwo, TWO));
        Expression e = new CombineDisjunctionsToIn().rule(or);
        assertEquals(or, e);
    }
    public void testMultipleIn() throws Exception {
        FieldAttribute fa = getFieldAttribute();
        Or firstOr = new Or(EMPTY, new In(EMPTY, fa, singletonList(ONE)), new In(EMPTY, fa, singletonList(TWO)));
        Or secondOr = new Or(EMPTY, firstOr, new In(EMPTY, fa, singletonList(THREE)));
        Expression e = new CombineDisjunctionsToIn().rule(secondOr);
        assertEquals(In.class, e.getClass());
        In in = (In) e;
        assertEquals(fa, in.value());
        assertThat(in.list(), contains(ONE, TWO, THREE));
    }
    public void testOrWithNonCombinableExpressions() throws Exception {
        FieldAttribute fa = getFieldAttribute();
        Or firstOr = new Or(EMPTY, new In(EMPTY, fa, singletonList(ONE)), lessThanOf(fa, TWO));
        Or secondOr = new Or(EMPTY, firstOr, new In(EMPTY, fa, singletonList(THREE)));
        Expression e = new CombineDisjunctionsToIn().rule(secondOr);
        assertEquals(Or.class, e.getClass());