[DSE]Enhance shorthening MemIntrinsic based on OverlapIntervals

Summary:
This change use the overlap interval map built from partial overwrite tracking to perform shortening MemIntrinsics.
Add test cases which was missing opportunities before.

Reviewers: hfinkel, eeckstein, mcrosier

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D21909

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275571 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed