fix wordgraph bug: 'wordgraph_check_coherence' will check wordgraph f…

[xiaowenth]

I notice function 'wordgraph_compaction_thesame_sub' has a bug.

1. before delete we(wordgraph), we's leftwords and rightwords will be added to wg, except for each of them is equal wg. that's to say selfloop is not allowed
2. when replacing we with wg by checking we's leftwords and rightwords, selfloop is allowed(otherwise leftwords or rightwords may pointer to a deleted we); so wordgraph_check_coherence will check failed because of the previous operations