### Description

Evals that introduce aliased can be simplified by extracting them into a project (and thus signaling there's no underlying processing).

The following eval:
`eval x = a + 1, y = x, z = y + 1, y = z, w = y + 1`
can be converted into:
`eval x = a + 1, z = a + 1 + 1, w = a + 1 + 1 | project x, z, z as y, w`

This helps fuse the project with other projections/aggs downstream.
x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/optimizer/LogicalPlanOptimizerTests.java
assertNullLiteral(rule.rule(new Substring(EMPTY, Literal.NULL, Literal.NULL, Literal.NULL)));
    }
    public void testPruneSortBeforeStats() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | sort emp_no
            | where emp_no > 10
            | stats x = sum(salary) by first_name""");
        var limit = as(plan, Limit.class);
        var stats = as(limit.child(), Aggregate.class);
        var filter = as(stats.child(), Filter.class);
        as(filter.child(), EsRelation.class);
    }
    public void testDontPruneSortWithLimitBeforeStats() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | sort emp_no
            | limit 100
            | stats x = sum(salary) by first_name""");
        var limit = as(plan, Limit.class);
        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);