[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

llvm-svn: 275571