[LSV] Insert stores at the right point.

Summary:
Previously, the insertion point for stores was the last instruction in
Chain *before calling getVectorizablePrefixEndIdx*.  Thus if
getVectorizablePrefixEndIdx didn't return Chain.size(), we still would
insert at the last instruction in Chain.

This patch changes our internal API a bit in an attempt to make it less
prone to this sort of error.  As a result, we end up recalculating the
Chain's boundary instructions, but I think worrying about the speed hit
of this is a premature optimization right now.

Reviewers: asbirlea, tstellarAMD

Subscribers: mzolotukhin, arsenm, llvm-commits

Differential Revision: https://reviews.llvm.org/D22534

llvm-svn: 276056