### 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 stats = as(limit.child(), Aggregate.class);
        var topN = as(stats.child(), TopN.class);
        as(topN.child(), EsRelation.class);
    }
    public void testCombineOrderBy() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | sort emp_no
            | sort salary""");
        var topN = as(plan, TopN.class);
        assertThat(orderNames(topN), contains("salary", "emp_no"));
        as(topN.child(), EsRelation.class);
    }
    public void testCombineOrderByThroughEval() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | sort emp_no
            | eval x = salary + 1
            | sort x""");
        var topN = as(plan, TopN.class);
        assertThat(orderNames(topN), contains("x", "emp_no"));
        var eval = as(topN.child(), Eval.class);
        as(eval.child(), EsRelation.class);
    }
    public void testCombineOrderByThroughEvalWithTwoDefs() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | sort emp_no
            | eval x = salary + 1, y = salary + 2
            | eval z = x * y