Add a sign extension lowering pass

[sbc100]

The llvm default dfeature set was recently added in https://reviews.llvm.org/D125728.

I'm working on an emscripten change that will automatically disabled these new features if the user targets older browsers: emscripten-core/emscripten#17690

We decided the simplest way to implement this "disabling" would be as a binaryen pass to lower away the new feature(s).

[sbc100]

@kripken maybe you fancy taking this on?

[kripken]

Sure, if no one else wants to, I can.