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

[ForNeVeR]

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.