Move control flow optimization to CmmCPS.

Unfortunately, I couldn't remove all incidences of runCmmContFlowOpt
from HscMain; in particular, there is a Cmm conversion testing
facility which may run with only control flow optimizations, which
I preserved the semantics of.  Given the state of the current
codegen, this code might be moribund anyway.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>