[ZoneAlgo] Handle non-StoreInst/LoadInst MemoryAccesses including memset.

Up to now ZoneAlgo considered array elements access by something else
than a LoadInst or StoreInst as not analyzable. This patch removes that
restriction by using the unknown ValInst to describe the written
content, repectively the element type's null value in case of memset.

Differential Revision: https://reviews.llvm.org/D37362

llvm-svn: 312630