Defined variable with relevant bounds incorrectly removed

In 2.7.1, 2.7.2, and 2.7.3, the following model:
```minizinc
var -100..100: a;
var -10..10: c = 2 * a;
```
flattens to the following flatzinc:
```minizinc
var -100..100: a:: output_var;
solve  satisfy;
```
which is incorrect. The flattened model has `a = -100` as a solution, but this would have `c = -200` which violates the bounds on `c` in the original model.