Simplify nested conditionals if possible

This change adds some simple optimizations to nested conditionals. In our codebase, we have some generated code that this applies to.

```js
x ? y ? z : a : a
// should be able to be safely converted to
x && y ? z : a
```
I don't have any benchmarks on how much of a benefit this provides in different circumstances, but I'd be happy to run some numbers if there's interest.

This is my first time contributing to Uglify; please let me know if there's anything I haven't done properly.