[DAG] Elide overlapping store

Summary:
Extend overlapping store elision to handle overwrites of stores by
larger stores.

Nontemporal tests have been modified to add memory dependencies to
prevent store elision.

Reviewers: craig.topper, rnk, t.p.northover

Subscribers: javed.absar, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D40969

llvm-svn: 321089