[tree-ssa] Loop analysis

[Jan Hubicka]

> Hi,
> 
> This patch allows the tree-ssa-branch to reuse the loop detection pass.
> For the moment it only initializes the loops structure.

Thanks,
can you please push the cfghooks bits into mainline?  Even when there
will be just one hook vector for RTL.  I would like to use it to cleanup
cfglayout and get the interface sanier.
> 
> I don't know whether it is worth to handle differences between representations
> in a cfg_hook structure, or simply guard the code with a boolean: cfg_at_rtl_level.
> What is difficult is to register static functions in the hook structure. 
> I think that the guarded code is the better solution.

Probably yes at the moment.  I will try to get this cleaned up so
make_forwarder_block for instance won't need to be IL specific.  Lets
see.

Honza