### 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
PropagateEquals rule = new PropagateEquals();
        Expression exp = rule.rule(new Or(EMPTY, eq, neq));
        assertEquals(TRUE, exp);
    }
    
    public void testPropagateEquals_VarEq2OrVarNeq5() {
        FieldAttribute fa = getFieldAttribute();
        Equals eq = equalsOf(fa, TWO);
        NotEquals neq = notEqualsOf(fa, FIVE);
        PropagateEquals rule = new PropagateEquals();
        Expression exp = rule.rule(new Or(EMPTY, eq, neq));
        assertEquals(NotEquals.class, exp.getClass());
        NotEquals ne = (NotEquals) exp;
        assertEquals(FIVE, ne.right());
    }
    
    public void testPropagateEquals_VarEq2OrVarRangeGt3Lt4OrVarGt2OrVarNe2() {
        FieldAttribute fa = getFieldAttribute();
        Equals eq = equalsOf(fa, TWO);
        Range range = rangeOf(fa, THREE, false, FOUR, false);
        GreaterThan gt = greaterThanOf(fa, TWO);
        NotEquals neq = notEqualsOf(fa, TWO);
        PropagateEquals rule = new PropagateEquals();
        Expression exp = rule.rule((Or) Predicates.combineOr(asList(eq, range, neq, gt)));
        assertEquals(TRUE, exp);
    }
    
    public void testPropagateEquals_ignoreDateTimeFields() {
        FieldAttribute fa = getFieldAttribute("a", DataTypes.DATETIME);
        Equals eq1 = equalsOf(fa, ONE);