Ranger was not allowing the exit block to be queried for range_on_entry 
or exit, for no good reason.  This removes that restriction.

Interestingly, it seems that when we calculate dominance info, GCC does 
not set the dominators for the EXIT_BLOCK?  I worked around it by 
starting with a single pred of the exit block for my queries, but as a 
result it doesn't support multiple exit blocks.

For the record:

   get_immediate_dominator (CDI_DOMINATORS, EXIT_BLOCK_PTR_FOR_FN (cfun))

returns NULL.   Is this actually working as intended?  It was unexpected 
on my part.

Bootstrapped on x86_64-pc-linux-gnu with no regressions.  Pushed.

Andrew