Add some notes about some tests that would be nice to have

Unfortunately, I don't know how to trick the compiler into generating
them (though it seems to be possible in the wild).  These are constructs
that exposed problems in the SSA transformation that are now fixed.
Additionally, these problems fail noisily, so regressions will at least
be noticeable.
hlint eta reduction
Add a helper function

This simple function is useful to strip off levels of casting to find
the underling value in use.
hlint pass
Remove the reference to note [CFG Construction]

I deleted it since its information was subsumed by documentation on
individual functions
Describe splitIntoBlocks a bit more, especially its results
Add an overview of the labeling algorithm
Expand the discussion a bit more