rework verify code
The new code has less options and variants, if you call irg_verify() verification is performed, regardless of DEBUG_libfirm, or NDEBUG being set and regardless of firm verification setting switches. On the other side no atomic dump and abort is performed anymore, the call has to do this, though an irg_assert_verify() function has been added as a convenience measure. There has also been a complete rewrite of the actual checking performed. The new code is simpler, should produce more consistent output and can report more than one error.