This major patch implements Join Points, as described in https://ghc.haskell.org/trac/ghc/wiki/SequentCore. You have to read that page, and especially the paper it links to, to understand what's going on; but it is very cool.
It's Luke Maurer's work, but done in close collaboration with Simon PJ.
This Phab is a squash-merge of wip/join-points branch of
http://github.com/lukemaurer/ghc. There are many, many interdependent changes.