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.
This will happen anyway, in the late phase because those are all peephole optimizations.  Where is matters is for more interesting optimizations, where the types are expected to be the same.