### 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/main/java/org/elasticsearch/xpack/esql/optimizer/LogicalPlanOptimizer.java
BooleanSimplification() {
            super();
        }
        @Override
        protected Expression maybeSimplifyNegatable(Expression e) {
            return null;
        }
    }
    static class PruneFilters extends OptimizerRules.PruneFilters {