Split name-minification into a separate pass from registerization.

Per comments in https://github.com/kripken/emscripten/pull/1981 this splits the name-minification logic into a separate pass from the registerization.

There are now two passes named "registerize" and "minifyNames" which should be applied in that order.  js_optimizer.py transparently splits "minifyNames" into separate calls to "minifyGlobals" and "minifyLocals" for the underlying js-optimizer.js, in the same way that it used to split "registerizeAndMinify" into "minifyGlobals" and "registerize".

I get a clean run of `python tests/runner.py` with this change, apart from `default.test_simd3` which fails for me both before and after the change.