[LSV] Don't assume that loads/stores appear in address order in the BB.

Summary:
getVectorizablePrefix previously didn't work properly in the face of
aliasing loads/stores.  It unwittingly assumed that the loads/stores
appeared in the BB in address order.  If they didn't, it would do the
wrong thing.

Reviewers: asbirlea, tstellarAMD

Subscribers: arsenm, llvm-commits, mzolotukhin

Differential Revision: https://reviews.llvm.org/D22535

llvm-svn: 276072