[LoadStoreVectorizer] Enable vectorization of stores in the presence of an aliasing...

[LoadStoreVectorizer] Enable vectorization of stores in the presence of an aliasing load

Summary:
The "getVectorizablePrefix" method would give up if it found an aliasing load for a store chain.
In practice, the aliasing load can be treated as a memory barrier and all stores that precede it
are a valid vectorizable prefix.
Issue found by volkan in D26962. Testcase is a pruned version of the one in the original patch.

Reviewers: jlebar, arsenm, tstellarAMD

Subscribers: mzolotukhin, wdng, nhaehnle, anna, volkan, llvm-commits

Differential Revision: https://reviews.llvm.org/D27008

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@287781 91177308-0d34-0410-b5e6-96231b3b80d8