Two small improvements to LiberateCase

Two small improvements to LiberateCase

Max Bolingbroke suggested these two small improvements to LiberateCase
(most of the size increase is comments :-)):

a) Do LiberateCase on small functions even if they are mutually recursive
   See Note [Small enough]

b) Don't do LiberateCase on functions for which it'd be fruitless,
   namely when a free varible is scrutinised *outside* the function
   See Note [Avoiding fruitless liberate-case]

There is virtually no effect on nofib, but Max tripped over cases
where it mattered slightly.