### 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
Range range = rangeOf(fa, ONE, false, THREE, false);
        GreaterThan gt = greaterThanOf(fa, L(0));
        NotEquals neq = notEqualsOf(fa, FOUR);
        PropagateEquals rule = new PropagateEquals();
        Expression and = Predicates.combineAnd(asList(eq, range, gt, neq));
        Expression exp = rule.rule((And) and);
        assertEquals(eq, exp);
    }
    
    public void testPropagateEquals_VarEq2OrVarGt1() {
        FieldAttribute fa = getFieldAttribute();
        Equals eq = equalsOf(fa, TWO);
        GreaterThan gt = greaterThanOf(fa, ONE);
        PropagateEquals rule = new PropagateEquals();
        Expression exp = rule.rule(new Or(EMPTY, eq, gt));
        assertEquals(gt, exp);
    }
    
    public void testPropagateEquals_VarEq2OrVarGte2() {
        FieldAttribute fa = getFieldAttribute();
        Equals eq = equalsOf(fa, TWO);
        GreaterThan gt = greaterThanOf(fa, TWO);
        PropagateEquals rule = new PropagateEquals();
        Expression exp = rule.rule(new Or(EMPTY, eq, gt));
        assertEquals(GreaterThanOrEqual.class, exp.getClass());
        GreaterThanOrEqual gte = (GreaterThanOrEqual) exp;
        assertEquals(TWO, gte.right());
    }
    
    public void testPropagateEquals_VarEq2OrVarLt3() {