delete_trivially_dead_insns usage

[Dan Nicolaescu <dann at godzilla dot ICS dot UCI dot EDU>]

Hi!

While trying to fix a bootstrap problem on the new-regalloc-branch I
saw that delete_trivially_dead_insns is used with the third argument
zero _everywhere_ in the tree. 

Using 1 for ra.c fixes a few problems. 

Question: what is the proper way to use delete_trivially_dead_insns:

do 
  delete_trivially_dead_insns (BLAH, BLAH, 0);
  find_basic_blocks (...);

  as it's done in toplev.c

or just use:
  delete_trivially_dead_insns (BLAH, BLAH, 0);

It would be also nice if there were some comments in toplev.c
saying which passes keep the CFG consistent...

Thanks.
        --dan