This is a follow-up task of JDK-8276219. 

ConnectionGraph::find_inst_mem() contains a self-recursion for MergeMemNode. It drills into one input of mergenode and tries to locate the memory node which has the exact alias_idx.  Once it returns, the result will be same from recursion. Therefore, it's not necessary to use self recursion in this case. we can reset the initial state of this function and respin. 

We can use a collection to remember all MergeMem Nodes and update them after then.