Hi,

in the header comment of function inverted_post_order_compute in 
cfganal.c we find:
...
    This function assumes that all blocks in the CFG are reachable
    from the ENTRY (but not necessarily from EXIT).
...

This patch checks that there are indeed no unreachable blocks when 
calling inverted_post_order_compute.

OK for trunk if bootstrap/regtest succeeds?

Thanks,
- Tom