Cesium leaves a lot of useless `conv.u` instructions everywhere

Take almost any code that works with arrays, for example, `CodeGenArrayTests.ArrayInitialization`, and you'll notice that Cesium leaves a useless `conv.u` instruction often. And none of those are actually needed, or at least I think they aren't.

I believe that Cesium should get a better stack model, and to only emit conversion instructions such as `conv.i` or `conv.u` when really required.