### 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
var filter = as(topN.child(), Filter.class);
        var eval = as(filter.child(), Eval.class);
        as(eval.child(), EsRelation.class);
    }
    public void testCombineMultipleOrderByAndLimits() {
        
        
        
        
        
        
        
        LogicalPlan plan = optimizedPlan("""
            from test
            | sort emp_no
            | rename salary as l
            | keep l, emp_no, first_name
            | sort l
            | limit 100
            | sort first_name
            | where l > 1
            | sort emp_no""");
        var keep = as(plan, Project.class);
        var topN = as(keep.child(), TopN.class);
        assertThat(orderNames(topN), contains("emp_no", "first_name"));
        var filter = as(topN.child(), Filter.class);
        var topN2 = as(filter.child(), TopN.class);
        assertThat(orderNames(topN2), contains("salary", "emp_no"));
        as(topN2.child(), EsRelation.class);
    }