WebAssembly is very different from LLVM IR, because it is essentially just the greatest common denominator of what existing JavaScript VMs handle in the backend

WebAssembly is very different from LLVM IR, because it is essentially just the greatest common denominator of what existing JavaScript VMs handle in the backend. For instance, WebAssembly allows only reducible control flow, has very limited support for function pointers, and a particularly nasty memory model (linear memory, which precludes any kind of non-trivial alias analysis and optimization). All of these things may change in the future (apart from the reducible control flow restriction), but, at the moment, the goal of WebAssembly is just to have a viable compilation target for C++ in the browser. Google was championing LLVM IR for this purpose in the form of pnacl, but that was not enough of a compromise to work on the web platform. :(