The last of the restructuring stuff before flattening basic-block.h.

5 new files, cfgbuild.h, cfgcleanup.h, cfgloopmanip.h, dominance.h, and 
ifcvt.h.  MOstly prototypes, but a few enums, #defines and structs were 
more appropriately located.

basic-block.h only includes  cfgbuild.h, cfgcleanup.h, and 
dominance.h.   Most of the prototypes for cfgloopmanip.h were actually 
in cfgloop.h, so cfgloopmanip.h is included from there instead.
"struct ce_if_block" was moved to the new ifcvt.h.  Other than ifcvt.c, 
the frv target was the only one which actually used the struct, so we 
include it there directly.

predict.h was also brought up to snuff to match predict.c. 
maybe_hot_count_p() was newly exported from predict.c so that the 
definitions of  cgraph_edge::maybe_hot_p() and 
cgraph_node::optimize_for_size_p() could be moved back to cgraph.[ch] 
where they belong.

Bootstraps on x86_64-unknown-linux-gnu, and testsuites are currently 
running.   I'll also do a full set of port builds over night...

assuming no issues, OK?

Andrew