Multiple improvements to CPS algorithm.

Multiple improvements to CPS algorithm.
These include:
 - Stack size detection now includes function arguments.
 - Stack size detection now avoids stack checks just because of
   the GC block.
 - A CmmCall followed by a CmmBranch will no longer generate an extra
   continuation consisting just of the brach.
 - Multiple CmmCall/CmmBranch pairs that all go to the same place
   will try to use the same continuation.  If they can't (because
   the return value signature is different), adaptor block are built.
 - Function entry statements are now in a separate block.
   (Fixed bug with branches to the entry block having unintended effects.)
 - Other changes that I can't recall right now.