Misc. progress

• Rename TightAssociative to MonoidReduce.
• Lower phis, reads, and writes in RT backend.
• Allow fork fission to not take in an explicit data label - in this case, implicitly the fission point is between the parallel section and the reduction cycle (this is the most common case).
• CleanMonoidReduces pass which makes outer forks around monoid reductions more easily parallelizable after fission.
• Fix GCM behavior w/ primitive constants / undefs.
• Make SimplifyCFG more aggressive with removing reduces (motivated by fission).
• Add UnforkifyOne pass, which only unforkifies the first loop it sees - useful for unforkifying outer loops.
• Multi-core dot product test that uses a bunch of the stuff in this MR.
• A whole bunch of fixes.