[DeadStoreElimination] Shorten beginning of memset overwritten by later stores

Summary: This change will shorten memset if the beginning of memset is overwritten by later stores.

Reviewers: hfinkel, eeckstein, dberlin, mcrosier

Subscribers: mgrang, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D18906

llvm-svn: 267197