[LSV] Don't move stores across may-load instrs, and loosen restrictions on moving loads.

Summary:
Previously we wouldn't move loads/stores across instructions that had
side-effects, where that was defined as may-write or may-throw.  But
this is not sufficiently restrictive: Stores can't safely be moved
across instructions that may load.

This patch also adds a DEBUG check that all instructions in our chain
are either loads or stores.

Reviewers: asbirlea

Subscribers: llvm-commits, jholewinski, arsenm, mzolotukhin

Differential Revision: https://reviews.llvm.org/D22547

llvm-svn: 276171