Substitute number equivalents when comparing to booleans

Resolves https://github.com/google/closure-compiler/issues/369.

I included `<`, `<=`, `>`, `>=` in addition to the `==` and `!=` operators requested originally.
Correct me if I am wrong, I believe this actually enables more optimizations.

(Kind of stupid) example: `0 <= true`

Currently, the Compiler does

```javascript
0 <= true
0 <= !0
```

With this change, the Compiler would do 

```javascript
0 <= true
0 <= 1
true
!0
```