Perform SSA -type of optimizations

(as @hzoo suggested in Slack)

c.f.: https://en.wikipedia.org/wiki/Static_single_assignment_form

```js
var x = 1;
var x = 2;
y = x;
```

We can determine that 1st statement is useless. Combined with variable inlining, we could shorten this to the ideal `y = 2`.