### Description

In cases where the same agg is used, instead of computing it multiple times, make another reference to it:
```
stats a = min(x), b = min(x) by z
```

should become
```
stats a = min(x) by z
eval b = a
keep a,b,z
```

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/optimizer/LogicalPlanOptimizerTests.java
assertTrue(filter.condition() instanceof Or);
        var or = (Or) filter.condition();
        assertTrue(or.left() instanceof LessThan);
        assertTrue(or.right() instanceof GreaterThan);
        var stats = as(filter.child(), Aggregate.class);
        assertTrue(stats.child() instanceof EsRelation);
    }
    public void testSelectivePushDownComplexFilterPastAgg() {
        
        LogicalPlan plan = optimizedPlan("""
            from test
            | stats x = count(1) by emp_no
            | where (emp_no < 3 or x > 9) and emp_no > 0""");
        var limit = as(plan, Limit.class);
        var filter = as(limit.child(), Filter.class);
        assertTrue(filter.condition() instanceof Or);
        var or = (Or) filter.condition();
        assertTrue(or.left() instanceof LessThan);
        assertTrue(or.right() instanceof GreaterThan);
        var stats = as(filter.child(), Aggregate.class);
        filter = as(stats.child(), Filter.class);
        assertTrue(filter.condition() instanceof GreaterThan);
        var gt = (GreaterThan) filter.condition();
        assertTrue(gt.left() instanceof FieldAttribute);
        assertEquals("emp_no", ((FieldAttribute) gt.left()).name());
        assertEquals(L(0), gt.right());
        assertTrue(filter.child() instanceof EsRelation);
    }
    public void testSelectivelyPushDownFilterPastEval() {