Warn when
   (a) a pattern bindings binds unlifted values
   (b) it has no top-level bang
   (c) the RHS has a *lifted* type

Clause (c) is new, argued for by Simon M

Eg     x# = 4# + 4#      -- No warning
       (# a,b #) = blah  -- No warning
       I# x = blah       -- Warning