### 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();
        Equals eq = equalsOf(fa, TWO);
        LessThan lt = lessThanOf(fa, THREE);
        PropagateEquals rule = new PropagateEquals();
        Expression exp = rule.rule(new Or(EMPTY, eq, lt));
        assertEquals(lt, exp);
    }
    
    public void testPropagateEquals_VarEq3OrVarLt3() {
        FieldAttribute fa = getFieldAttribute();
        Equals eq = equalsOf(fa, THREE);
        LessThan lt = lessThanOf(fa, THREE);
        PropagateEquals rule = new PropagateEquals();
        Expression exp = rule.rule(new Or(EMPTY, eq, lt));
        assertEquals(LessThanOrEqual.class, exp.getClass());
        LessThanOrEqual lte = (LessThanOrEqual) exp;
        assertEquals(THREE, lte.right());
    }
    
    public void testPropagateEquals_VarEq2OrVarRangeGt1Lt3() {
        FieldAttribute fa = getFieldAttribute();
        Equals eq = equalsOf(fa, TWO);
        Range range = rangeOf(fa, ONE, false, THREE, false);
        PropagateEquals rule = new PropagateEquals();
        Expression exp = rule.rule(new Or(EMPTY, eq, range));
        assertEquals(range, exp);
    }
    
    public void testPropagateEquals_VarEq2OrVarRangeGt2Lt3() {
        FieldAttribute fa = getFieldAttribute();