[vector] RFR: Fix crash at compile.cpp:2695

[Vladimir Ivanov]

http://cr.openjdk.java.net/~vlivanov/panama/vector/repeated_expansion/webrev.00/

Merge-through-phis can lead to a situation when VectorAllocateBox is 
shared between multiple VectorBox nodes. It happens when there are other 
uses of a Phi which participates in the merge. In that case it's safe to 
skip the expansion: the value stored into the already expanded box 
should be the same.

Best regards,
Vladimir Ivanov