Emscripten generates code that allows the caller to provide the Wasm
module (thorugh Module.instantiateWasm), with a fallback in case
.instantiateWasm is not provided. We always define instantiateWasm, so
we can hard-code the check and let our dead code elimination logic
remove the unused fallback.
This commit also improved the dead code elimination logic so that if
a function declaration becomes unused as a result of removing dead
code, the function itself is removed.
