[DAG] Elide overlapping stores

Relanding after fixing handling of pre-indexed memory operations in
BaseIndexOffset analysis (r322003).

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

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

Subscribers: javed.absar, hiraditya, llvm-commits

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

llvm-svn: 322085