Implement first-half/second-half splitting of stage 2 optimizations.

This change adds code to DefaultPassConfig for enabling splitting of stage 2 optimizations.

I'm also adding a unit test to ensure that running the two halves of stage 2 result in the same output code as running all of stage 2 in one go.

PiperOrigin-RevId: 718096864

Author: rahul-kamat