### 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
var plan = plan("""
            from test
            | stats s = sum(salary) by last_name, first_name
            | keep s, last_name, first_name
            """);
        var limit = as(plan, Limit.class);
        var agg = as(limit.child(), Aggregate.class);
        assertThat(Expressions.names(agg.aggregates()), contains("s", "last_name", "first_name"));
        assertThat(Expressions.names(agg.groupings()), contains("last_name", "first_name"));
    }
    public void testQlComparisonOptimizationsApply() {
        var plan = plan("""
            from test
            | where (1 + 4) < salary
            """);
        var limit = as(plan, Limit.class);
        var filter = as(limit.child(), Filter.class);
        
        var condition = as(filter.condition(), GreaterThan.class);
        assertThat(Expressions.name(condition.left()), equalTo("salary"));
        assertThat(Expressions.name(condition.right()), equalTo("1 + 4"));
        var con = as(condition.right(), Literal.class);
        assertThat(con.value(), equalTo(5));
    }
    public void testCombineProjectionWithPruning() {
        var plan = plan("""
            from test
            | rename first_name as x
            | keep x, salary, last_name
            | stats count(salary) by x