[LSV] Vectorize up to side-effecting instructions.

Summary:
Previously if we had a chain that contained a side-effecting
instruction, we wouldn't vectorize it at all.  Now we'll vectorize
everything that comes before the side-effecting instruction.

Reviewers: asbirlea

Subscribers: arsenm, jholewinski, llvm-commits, mzolotukhin

Differential Revision: https://reviews.llvm.org/D22536

llvm-svn: 276170