In InferReactivePlaces, we already account for reactively controlled values:
where a value is never assigned a non-reactive value, but _which_ value is
assigned is based on a reactive condition (the test conditions of an if, switch,
loop, etc).
This PR extends that reactively-controlled inference to mutation that is
conditioned upon a reactive value. From the test case:
```javascript
let x = [];
if (props.cond) {
// This mutation has no reactive inputs.
// *But* the mutation conditionally occurs based on props.cond which is reactive
x.push(1);
}
let y = false;
if (x[0]) { // therefore the value observed here is reactive
y = true;
}
// so the value of y here is reactive via the reactive control dependency x[0]
return [y];
```