Request for reviews (XS): 6812217: Base memory of MergeMem node violates assert during killing expanded AllocateArray node

[Vladimir Kozlov]

http://cr.openjdk.java.net/~kvn/6812217/webrev

Fixed 6812217: Base memory of MergeMem node violates assert during killing 
expanded AllocateArray node

The assert in MergeMemNode::memory_at() misses the case when address is TOP.