This patch flattens cfgloop.h.  cfgloopmanip.h is utilized by numerous 
files which include cfgloop.h, so it seems to make sense to leave it 
here. The other 3 header files are going to form part of backend.h, so 
take them out.

I didn't bother posting the changes to all the source files since 
aggregating with backend.h would undo every one.

Bootstraps from scratch on x86_64-unknown-linux-gnu with no new 
regressions. Also compiles all the files in config-list.mk.