variable could be assigned at declaration

I believe the output from advanced mode could be (mildly) optimised from `var a;a=window.setTimeout;` to `var a=window.setTimeout;` without causing any issues.

The full input file was:
```
// ==ClosureCompiler==
// @compilation_level ADVANCED_OPTIMIZATIONS
// @output_file_name default.js
// ==/ClosureCompiler==

function hello(name) {
alert('Hello, ' + name);
}

var timer;
if( true ){ // or a truthy variable
timer = window['setTimeout'];
}

timer(function(){ hello('New user'); },100);
timer(function(){ hello('something else'); },100);
```
producing
```
var a;a=window.setTimeout;a(function(){alert("Hello, New user")},100);a(function(){alert("Hello, something else")},100);
```