Added stack checks to the CPS algorithm

Added stack checks to the CPS algorithm

This eliminates one of the panics introduced by
the previous patch:
'First pass at implementing info tables for CPS'

The other panic introduced by that patch still remains.
It was due to the need to convert from a
  ContinuationInfo to a CmmInfo.
  (codeGen/CgInfoTbls.hs:emitClosureCodeAndInfoTable)
  (codeGen/CgInfoTbls.hs:emitReturnTarget)