Hi,

In verify_loop_structure, we stop checking the latch once we find that 
it's NULL.

This patch tries a bit harder:
- if !LOOPS_MAY_HAVE_MULTIPLE_LATCHES, we don't allow a NULL latch
- if LOOPS_MAY_HAVE_MULTIPLE_LATCHES, we check that indeed there's no
   single loop latch.

As a consequence of adding this check, I needed to fix 
expand_omp_for_generic, which missed an initialization of a loop latch.

Bootstrapped and reg-tested on x86_64.

OK for stage3 trunk?

Thanks,
- Tom