RFR: 8280541: remove self-recursion of ConnectionGraph::find_inst_mem() [v2]

[Xin Liu]

> This is a follow-up task of JDK-8276219.
> 
> ConnectionGraph::find_inst_mem() contains a self-recursion for MergeMemNode.
> It drills down into one input of MergeMemNode and tries to locate the memory node
> which has the exact alias_idx. Once it returns, the result won't change from
> recursion. Therefore, it's not necessary to use 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. 
> 
> This patch also makes a cleanup in MergeMemNode::memory_at(). C is not in use in
> that function.

Xin Liu has updated the pull request incrementally with one additional commit since the last revision:

  Add an assertion in debug build.
  
  This patch ensures the result is same as recursion.

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7204/files
  - new: https://git.openjdk.java.net/jdk/pull/7204/files/ae0d0217..b9d40925

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7204&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7204&range=00-01

  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7204.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7204/head:pull/7204

PR: https://git.openjdk.java.net/jdk/pull/7204