[DSE] Teach the pass about partial overwrite of atomic memory intrinsics

Summary:
This change teaches DSE that the atomic memory intrinsics can be overwriten
partially in the same way as the non-atomic forms. Specifically, that the
atomic memcpy & memset can be shortened at the end and that the atomic memset
can be shortened at the beginning, if they partially overwritten
by later stores.

Reviewers: mkazantsev, skatkov, apilipenko, efriedma, rsmith, spatel, filcab, sanjoy

Reviewed By: efriedma

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D45584

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@331991 91177308-0d34-0410-b5e6-96231b3b80d8