[NewOptimizer] Union Split during Inlining
This adds union splitting optimizations to the inliner.
This works a bit differently from the old optimizer, which
did union splitting on the bail out path. Instead, we try
to run the full inliner on any union split case that's a
dispatch tuple (and record what to do in that case). Doing
that allows an effective resolution of #23338, though this
PR doesn't quite do that yet, since some further (minor) fixes
are needed.
