Add first_block and last_block to loops structure

[Michael Hayes]

Jan Hubicka writes:

 > This patch adds two new fields (first_block and last_block) to the loop
 > structure.  They can be used by passes that tread loop as interval in
 > the insn chain such as alignment code in shorten_branches.
 > Also is usefull for searching of LOOP notes, because they ought to appear
 > near the first_block and last_block.
 > 
 > I am calculating the info in flow_loop_nodes_find, because it is readily
 > available there. If you know about better solution, please let me
 > know.

While this is useful in the interim I feel that these fields should be
calculated by a separate function that is called only when required.
This keeps the original function easier to understand and when the
remaining passes get converted to use the CFG, it can be easily
deleted.

Michael.